You can use anything you bought, without crediting it. 

For freebies, the stuff that gives permission for commercial use is okay.  While crediting freebies in the gallery here is considered good manners, it's not really practical when selling prints.

I don't think selling prints on eBay is going to earn you a lot of money.  Check and see what other art is going for...if it's going.

IME, you might have better luck selling graphic sets online (web graphics, brushes, tubes, scrapbooking graphics, etc.) than selling prints on eBay.

You might consider some place like Zazzle, which will handle the printing for you.  Less risk that way.