well you technically aren't going to be making a profit as the money you get for a used tv is most likely less than 10% of what you paid for it.
how much you get for it would be determined by how old the television is and what shape it is in. if we are talking about a projection or tube type then you will not get much at all. if we are talking about a year old lcd then you might make 25-30%... maybe.
craigslist, local pawn shops, newspaper ads......are your best bet