To be fair, it was initially magazines… I started taking a stall at what was then the Whiplash Market in the mid-90s, selling copies of the magazines I worked for. It was fun, but it was predominantly pocket money: I had a full-time day job (producing said magazines) and the publishers were happy to let me have a bundle of each month’s edition; they regarded it as extra PR.
A little later on, they started allowing me the run of the review cupboard – long before 50 Shades, there was a mini-explosion in the world of erotic fiction, with several of the big publishing houses setting up an erotica imprint and a couple of smaller publishers dealing solely with sex stuff. Books were frequently sent in to the mags for (hopefully) glowing reviews, and then they just sat in the cupboard, taking up space, so there came a point when the company was happy to let me cart away armfuls. And then there was the Guild, which I ran with a couple of friends, and the two anthologies we produced…
Over the years, good sources of books to sell on came and went; I diversified into t-shirts, with an artist friend, and those did quite well for a while but (this being the days before print-on-demand existed for anything, much) it was difficult to keep enough of a range of sizes and designs in stock at any given time. Later, I added badges and 
keyrings to my stall, and at one point got quite fanciful in terms of printed items: door stickers, fridge magnets and even temporary tattoos. But there were always books – mostly pre-loved but, in the brief era of me actually having my own little shop, selected from a good distributor.
There were a few years where my stallholding was really just ticking over; I was doing more events of a non-adult nature and expanding the badges/magnets/stickers rather more, but the problem with selling that kind of stock is that every individual item has a profit margin of pennies, and a sale price of less than £5, so you have to shift a lot in terms of volume. Bad weather or a badly-promoted event can lead to, well, losing a lot more money than you can afford and, however much fun you might have, there comes a time when it’s wholly unsustainable.
Nowadays, it’s not just my writing career that’s having its second innings, but the stallholding side is thriving, as well. I’ve got a fair range of anthologies featuring my own work and that of other people, which I love promoting. I’ve got a few contacts who produce good non-fiction on sex and BDSM themes who will generally give me a good wholesale price. And I’ve got a couple of sources of excellent pre-loved goodies, the sourcing and choosing of which is nearly as much fun for me as it might be to a potential customer finding a long-wanted rarity on the stall.
So if there’s a fetish fair near you, pop down and say hello. It might be the BBB, or it might be the LAM, or it might be Twisted in Brighton… or anywhere else that will have me.