no, they don't sell used sex toys back to people.
this isn't an issue for you as far as getting a std from a sex toy that only you have used.
you can get bacterial and fungal infections from sex toys. be sure to properly clean them and store them as recommended on the booklets you got with them. also when they start to show signs of breaking down, time to throw them out and get new ones :)
also remember to use lube so you don't get irritated!!!