There's no way to really know. Unless you use a 3rd party to act as an escrow, there is always a risk one party to a deal does not come through.
I'm not sure what exactly you are referring to, but if this is a contest, entry should be free. I think it's always a little dubious by default to run a contest requiring people to buy something for a "chance to win". That sets off my alarm bells at the outset.