Hi Shark
I guess if you need the gun in a real short amount of time, then your options are limited. It seems a shame to spend $100 on a gun that may cause you lots of problems though. Wallmart may have a returns policy, but do you really want to be returning a gun every few months when they break?
If it was me, I'd ask if anyone at the event I was going to had a spare gun they were prepared to lend. That way you get to try out different guns and may find one you really like and want to save up for. Also, if you're a tourney sort of 'baller, lots of players have spare guns they are looking to sell quite cheaply. Although these are second hand, you often get far better guns for the price than new, and they have usually been well looked after, seen little use and sometimes tricked out a bit.
If you absolutely, positively have to have a marker for this event and you aren't fussy, then get something really cheap, l;ike a pump pistol. You'll be out-gunned, but this will force you to play better as a result and you won't waste so much on a marker that will fail a few months down the line (not implying that JT's will, JT-lovers
), then you can save up for something that will be reliable, accurate and upgradeable which should last you a good long while.