Pretty much sums it up... Best case scenario the kit will be perfectly fine, worst case scenario... Well, just use your imagination. It all depends on factors that you can't provide right now so any yays/nays you get are just going to be based on personal opinion or experience but your mileage may certainly vary.

I bought my TN tuner kit used, but it had a brand new turbo - which to me is the part I would worry about most with a used kit. The rest is 90% piping.

To give yourself the best possible chance for success with a used kit, I would (if it is reasonable) check it out in person, or have a shop inspect it. And buy from a reputable member.

Good luck!