It is very difficult to make a living purely by selling prints. However it does get easier the more your name and work are known. I think that many people still find it surprising that photographic prints sell for more than $100, and are shocked that they can sell in the thousands of dollars ( and more). However it's gotten easy for people to buy your work online, never having seen anything more than a jpeg if they can Google your name and see that you are an established artist. For others selling prints on ebay not having that recognition makes it a far harder sale.
The venue where art is sold has a huge effect on the perceived value of the work and maybe that is one area where having your work sold on ebay could hurt sales. But it all depends on the reputation of the artist and the quality of the work. Personally I would be hesitant to buy even an Ansel Adams or Weston print on Ebay unless I knew that the seller was a well regarded collector or gallery. So much that affects the perception of a print's value is also based on the provenance of the print. This does not apply to lower priced prints, but once people are forking over significant amounts of money, they want the assurance of a brick and mortar gallery.
I think eBay can work for an established artist selling a second line of their work, inkjet prints at a modest price instead of selling their traditional prints at a far higher price. The risk of buying an inkjet print, of an image they like for $50 or $100 bucks, by an established artist who sells silver prints for many for times that price, is a small risk. It might be a good business model for a photographer.
Posters of my work routinely appear on eBay which is something that I do not appreciate because they tend to advertise them as "art prints" or "art photographs" and I am always concerned that it may confuse people or somehow devalue my silver prints. I have no idea how well the posters sell online, but they have been selling them there for years. I only hope they bought them from a licensed reseller and did not print them off original posters themselves.