There's a 100 different things to look out for when buying a MacBook off of eBay. Look at the sellers feedback and ask questions. Find out how it was used, what it was used for, and what condition the screen is in. If they don't discuss the body of the MacBook Pro, ask how it looks. If the price is too good to be true, then it probably is. There are some good sellers on there and generally the ones in mint or excellent condition command more of a price than ones that have been around the block a few times.
Honestly eBay is hit or miss. I hate when people describe things as "Mint" only for it to show up with scratches and scrapes on it. I swear some people don't know what "Mint" means. To me it means it looks brand new just like when it was opened from the box.
I purchased a 2015 about 6 months ago from a seller who described theirs as "In mint condition." When it arrived, it was dirty, had scratches on it, and had some nicks on the screen. When I questioned the seller over this his definition of "Mint" was far different than mine. He felt it meant that there were no dents in the body and that the screen had no noticeable scratches while it was on. While it didn't have either of those, there were two tiny chips in the screen and the outside of the MacBook had scratches all over it. I paid a little extra because of how he described it, and he was careful not to show any of the scratches in the listing. He refused a refund so I had to file a claim with eBay and easily won my money back. Point being, make sure you ask questions from the seller. I ended up just purchasing directly from Apple.
And from a specs standpoint, I'm partial to the 2.8GHz mid 2015 with External Graphics. Just my opinion though.