Unless the shop you won from reports it to IRS I don't know how they'd (the IRS) know. If they did report it you should get some type of 1099 ish form. If they don't, You are supposed to be honest and report it anyway. Generally the retail value of the item is considered to be the amount of income to you if the prize wasn't cash.
As far as sales tax goes, If you win a gift cert, you'll pay sales tax on purchase the same as if you paid cash, if they charge tax.
Good luck :D :D