I used to run a salon and my biggest pet peeve was ebay. They sell the lotions for less than the salons can even get them for.
That being said, it is likely that you will buy a lotion that is expired. Some of the ingredients may be inactive or not work as well. You will also be supporting illegal practices, as these people are obviously stealing or obtaining the lotions in ways they are not supposed to be, and are not tanning salons... In the beauty industry you are supposed to be a salon to sell the products from certain brands.
If I were you I would keep my eyes open for coupons to local salons so you know your product is not expired and just not go for a top of the line product. The less expensive brands can make some good product. Unless you want the anti-aging, cellulite covering special skin care lotions, the cheap ones will keep your skin hydrated with is the primary goal. We sold a lot of that whole line with the names like "sex kitten" and "sex pot" and they were mostly to repeat customers. I think it was Carribean gold or something? They changed packaging and the bottles all say "cheers" on them now.