Conventions, newsletters, mailing lists, lists of potential buyers to contact personally, collectors contacting him and asking...
Having observed people in a fanatical collecting hobby (vintage pens) through the insistence of a friend of mine, I get the impression that people involved in such collecting often strongly dislike eBay and similar sites. eBay, for all its faults, creates some level of transparency and openness in the market, and allows everyone to buy, sell, and appraise. For what are essentially fragile but persistent bubbles, transparency is bad.
Dealers want to be able to tell stories about pieces and their rarity, without a large market to let buyers verify them, and want to be able to hide sales, letting them inflate prices as much as possible. They certainly don't want anyone to be able to compare prices. Buyers want to be able to get special access to sales of pieces (or what they think is special access), or want to find people who don't know the value of what they have.
It's interesting hearing people in the hobby talk about these things. Dealers will rant about eBay and its dangers, or about buyers who "don't want to pay enough" (meaning they didn't sell that piece at all). Buyers will talk about how so-and-so contacted them first about a new piece, or about the amazing rarity of pieces that nevertheless seem to show up pretty often, or about how values appreciate with time. Unsurprisingly, doing painstaking research on auction sales over the last several decades, my friend found that values of most pieces did not even outpace inflation. Somehow, this didn't dissuade him. The hobby/bubble in its current form, though, is dying, and that's a good thing.
This is even more the case for collecting bubbles that are based around items currently being manufactured, as there is always the possibility of just creating more, and that risk is highest when everyone knows what is going on. In this case, for example, the manufacturer was easily able to destroy the dealer's business, but did so because the pieces were being very publicly sold. Had he instead kept everything quiet, and sold privately, he could have kept up what was essentially a scam for much longer.