If you are going to Grand Prix: Columbus with an eye for subsidizing your trip by trading, put an eye on how you can build a binder that any Legacy player will flip over. There is a specific attribute of the Legacy format that makes binder assembly simple, and that is the standardization of sideboard cards. More than any other format, you can predict to see the same cards, over and over, in sideboards across decks. You can benefit by making a binder just for people to complete their sideboard with, and then advertising that you have a specific binder for it. Consider these two scenarios:

Spike Player: "Do you have any Extirpates?"

You: "Umm, yes I do, I think they're in my black binder over here. Let me get that out... okay, here it is."

versus:

Spike Player: "Do you have Extirpate?"

You: "sure, they're in this binder. I put all the sideboard graveyard hate in one area in this, so flip through and see if there's anything else you need, too. I've got a lot of interesting options in it, you might get some inspiration."

Spike Player: "Okay, here are the Extirpates... maybe I need these Tormod's Crypts, too... oh, and how much do you value this Perish at"

By putting relevant cards near your trade fodder, you make it more attractive for the player looking to round out their deck to get what they need. They see other cards that are attractive and are more inclined to jump on them. Also, they don't have to look around multiple binders for cards that they require. This method is geared toward clearing out low-value, highly desired cards like Relic of Progenitus rather than getting the wheeling-and-dealing trades that you usually make, where both people are just browsing around for a variety of cards that interest them.

You also have an advantage as a trader because you can capitalize on site-specific needs of players. They want a card, and they cannot play without it. They may have realized on the drive up or in the hotel room the night before that they have to rework what they will be boarding. If they need an uncommon for their sideboard, they are unlikely to have packed it or find it in a trader's binder. So now, our player in search of, say, Krosan Grip, has to either pay $5 from a dealer for one or go without it if they cannot trade from someone. Few people will have those Grips in their binder (even though they are worth more than three pages of junk rares). Then they will find you, who will happily trade them for $5 in value to them. They're happy because they don't have to blow real money on cards, and you can make a healthy margin by having those uncommons.

Let's look at what cards are good to have in that binder. I strongly suggest organizing by theme (graveyard hate, creature kill, etc.) instead of by color. If I am looking for Deathmark, I might decide that I needPath to Exile instead, and I will probably only realize this when I see them sitting next to each other in your binder. The following list, then, is organized by theme and not color.

Graveyard Hate Cards

Graveyard hate is almost universal in Legacy, so you can always find ways to get rid of these cards. You can also use this part of your binder to trade four-packs of cards like Relic of Progenitus that are usually not worth the effort of trading. Another almost-universal theme of Legacy sideboards is that they rarely pack enough graveyard hate. Why not suggest to the person you are trading with that they consider adding in a Ravenous Trap or another Tormod's Crypt? The latter, especially, can be found for $1 in online stores, but can command $3-4 in trader cases at an event like the Grand Prix. It is not out of the question to buy large quantities of the Crypt expressly for trading value at the Grand Prix. Here is a list of suggested cards to sleeve up in this section:

Tormod's Crypt

Relic of Progenitus

Extirpate

Leyline of the Void

Planar Void

Morningtide

Ravenous Trap

Artifact and Enchantment Hate

Leading the pack in this category is Krosan Grip, the most widely-played sideboard card in Legacy. Beyond The Kung-fu Grip, though, is a multitude of other good hate cards that you can stock in your binder. A combination deck might want a blue bounce spell instead of a green instant, so it pays to have a wide variety of these on hand. Here's my suggested list:

Krosan Grip

Ancient Grudge

Qasali Pridemage

Ray of Revelation

Reverent Silence

Pithing Needle

Null Rod

Wipe Away

Chain of Vapor

Do you see how many of these are commons and uncommons? Remember, location is on your side!

Nonbasic Land Hate

Players often neglect cards like Blood Moon in their sideboard, and you can remind them how good it is. Many decks can run nonbasic hate, and there are three primary cards (aside from Wasteland that accomplish it. They are:

Back to Basics

Magus of the Moon]

Blood Moon

You can also use this opportunity to suggest cards likeTsabo's Web to players. If you have junk rares like the Web, this is a good way and a good time to dispose of them.

Creature Hate

Many decks that aim to win through attacking will pack in blowout cards in the sideboard to make their match better. Creature hate is also popular in slower decks like the UW Thopters deck, intending to use them to stall for time. Consider that the cards are almost always sweepers and not one-for-ones, since the cardpool is so deep that you can run hyperefficient answers. I won't include Umezawa's Jitte in this list because it is the GP promo and everyone will be eager to trade them.  Here's my go-to list:

Propaganda

Perish

Firespout

Sower of Temptation

Engineered Explosives

Humility

Path to Exile

Reprisal

Again, note how many of these are uncommons or even commons! You can trade them at a dollar or more to people who are in need at the time.

Combo Hate

And, believe it or not, combination decks are not dead, even though Mystical Tutor is. There is a broad range of options for combo hate and you can offer a good selection to people. Consider adding in the following:

Ethersworn Canonist

Gaddock Teeg

Mindbreak Trap

Arcane Laboratory

Rule of Law

Trinisphere

Sphere of Resistance

Chalice of the Void

Meddling Mage

A Special Word on Null Rod

Null Rod cancels all sorts of things - graveyard hating artifacts, Thopter Foundry, Sensei's Divining Top, equipment activations and more. They are currently going for around $7-10 and I expect them to be the "it" card for sideboards at the Grand Prix. If you have any for trade, make sure they are visible and let folks know you have them. You can command a healthy margin on them at the event site.

Pulling it Together

While this is intended for a Legacy tournament, you can apply the principles to New Extended or Standard tournaments, especially when there are valuable sideboard cards to be had. It is a lot harder to make your margins when you are trading small change, in print commons for people, though. You can profit most when there are uncommons that people want, cards like Propaganda, that are not in binders because they are not rare. Make sure you know where everything is in your sideboard binder; the guy who is poring through your collection might ask if you randomly have a Back to Basics and you'll need to know where it is.

Quick Recap:

• Assemble a special binder of sideboard cards
• Include older uncommons and commons that dealers will charge too much for
• Advertise to players that you have a sideboard binder, telling them they might get inspiration just flipping through it
• Organize the binder by genre of card, not by color
• Know where you moved cards from so you can quickly move them back after the event

Happy trading!