As mentioned it is bring yor own tank. If you do not have a set up this can been done very cheaply. 10gal tanks are about $10 now, a heater and powerhead, some egg crate to put the frags on and some extention cords/outlet strip you are at about $50 and only need a light, There are plenty of options here. Personally I like the t-5 options out there. Easy to move around and set up, plenty of light and very useful. You can get your self into a very nice 24" unit for a couple hundred bucks.
If this is your 1st event and you plan on going to more, and investment of a couple hundred bucks will easily be recouped at this event, may take 2 events to do depending on what you have to sell.
If possible the best thing to do is look at the space you have at home, and you set up a frag tank? If so what would it be? Plan your purchase to be something you can run at home for your frags, then take down and haul away to a show. This way your invetment is working for you continuly.
Selling at frag swaps can be a great way to expand your hobby and collection. I know several fraggers that make a grand at an event. That covers for them a big swap set up, custom tank and lights and ect ect. I know others who sell everything for $15, use a basic set up and still clean house. And still more who put thier frags in bagies and sell them that way and sell a lot. This baggie method is not recommended for a two day event like FragFest though.
I have a couple prefered ways depending on the event, with good pics of your colrals the baggiee method does very well for the amount of work it takes to get ready, but again not for 2 day events.
Bio cubes or something of the sort are great as well. As Tom pointed out it is all self contained and easy to set up.
At some shows I bring a 2x4' tank and go to town. This goes require a lot of water and prepwork, but depe3nding on the show it too is well worth it.
There are so many ways to go about this. Really I think what the best advice is would be to look at your home system and see if you can add afrag tank to it. If so plan off of that and use that tank at the swaps.
Best of luck!