I think sometimes good purchases can be made on eBay, and I have done so in the past when my finances didn't allow fresh film, which was often.

After a few years of shooting expired film with varying results, I decided it was false economy to continue. So I forced myself to shoot less film and always buy fresh. It costs more initially, but the results are consistent, and I know what to expect.

What I have seen lately, though, is that when I do pop into the auction site, just for giggles, I see prices of film that are pretty high! It's close to that of Freestyle, especially if you go for their inexpensive private label film, like Legacy Pro or the 'faux Kodak' stuff. So to me it doesn't make sense anymore. It is a bit of a crap shoot to buy expired film.

To answer the original question - I think it does maybe hurt the fresh film market, but it's natural that it occurs. You might argue that it would be a waste to just dispose of the film without using it, even if it's old. And if it's cheaper, obviously some photographers don't mind paying less for something that is a bit of a risk quality wise. Supply and demand. The old expired film wouldn't be sold unless there was a market for it.
And, it's good with competition for the fresh film suppliers. Without competition the prices might be even higher than they are on fresh film.

Buy what you can afford, (and that could mean to shoot less and still buy fresh), and go make some pictures!