An interesting initiative here:
Aquaman is proposing a bookshop at the final BronyCon next year. This means an actual, physical place selling actual, physical books. I have no idea whether it will work out, but the comments contain an impressive number of high-powered ponyfic people, so it seems to have a fair chance. If you want to know all the ins and outs and whys and wherefores, read the Aquaman blog I linked to. It's long, but it's readable enough for that not to be a big problem.
This really underlines how much of a gulf there is between drawing and writing in terms of artists/authors selling their stuff. There are probably thousands of visual artists who at least make the occasional few quid through PayPal. The number of writers who do the same is tiny, restricted to a few good enough to charge for commissions or editing work and an overlapping few good enough for it to be worth their making Lulu (or whatever) editions.
Now, if this BronyCon Bookstore idea ends up bringing a gigantic C&D down on Fimfiction, it will go down as one of the fandom's legendary self-inflicted disasters. Some people might now be jaded or cynical enough not to care, but many, many more still do. But it won't. As pointed out in the blog, if that was going to happen it would have done so long since, and I'd add especially given some of the things that Fimfiction allows in stories hosted there. So I wish it all the best.
For what it's worth, I have no problem at all with ponyfic authors selling books of their work. Given that I have a folder full of fan art purchased at conventions, plus several non-licensed plushies, I'd be a hypocrite if I said anything else. It's never been relevant to me: even if I were good enough, I wouldn't do it myself, as I don't want to make
any money from my ponyfic – I decided way back in 2012 that I would opt out of any AdSense-type programmes Fimfiction might introduce in the future. But if others do, then in a world of paid-for fan art and plushies I don't see why not.
On a slightly wider topic, here's a paraphrased comment I saw very recently, making a suggestion as to an additional reason why fanfic doesn't generally sell: "Fan art prints are generally cheaper than non-fandom art prints. Fan books are generally more expensive than non-fandom books." There are exceptions to all of those, but I thought it was quite perceptive: here in the UK, you wouldn't expect a good-quality paperback novel to cost more than £10. A ponyfic self-print will, so it's more of a luxury investment, whereas a fan art print at a fiver is not.