A custom knife is a knife that is made to the exact specifications of the customer, but I know what you mean.
Even the most hardcore knife enthusiast might miss out on a bunch of knives if there weren't any dealer sites -- dealers allow anyone to browse their site, and hundreds, even thousands of different custom makers and knives. Sure, you could argue that a directory with pictures of knives from various makers would be nice, but it wouldn't be the same as a store you can buy from, and it wouldn't result in nearly the same amount of custom maker and knife knowledge to most users. Without dealers, so many knife makers would most likely go unnoticed, because people would simply not hear about them. Getting the products into the customers hand would only go so far -- even if a couple of customers bought hundreds of new knives every month or week and talked about them (Ie, so they'd "advertise" various different knives, and custom makers.), getting their viewers from being interested in any specific knife out of all those, and onto the custom makers site, to making a buy, would be a significantly more complicated process than the current process; directly browsing a site with all those knives directly for sale.
It'd also cause a bit of information overload, since, as opposed to simply browsing a site for the knives, people would have to browse all those reviews, pick a knife, find the site address for the custom maker, and either buy it from the custom makers site or email the custom maker.
I think the custom makers in question are responsible for this, though, not the sellers. They shouldn't get back ordered for several years, they shouldn't sell in bulk at a deep discount (If they can, then this would mean that they are actually production knives, and shouldn't command a premium price.). Of course, in some cases this is completely understandable -- they need to generate some P.R and get sales. But if you're back ordered, there's no reason why you should do so. I think that, in most cases, there should be a 1 knife per customer rule. In any case, they shouldn't prioritize bulk orders, or show orders, if they are backlogged. If indeed they are backlogged, they should, in most cases (And in my opinion, of course.), focus on whoever ordered first.
However, most of this usually only applies to already well-established custom makers.
So.. yeah, custom knife dealers are, basically, necessary. They can be good, or bad. Custom makers don't have to sell anything to them, but it can be easier to sell in bulk to a single person/company. But this is a choice made by the individual custom makers. If they are backed up for several years and suddenly a bunch of knives appear on a custom knife dealer site, keep in mind that the custom knife maker might have ordered a long time ago. But this is why I think that custom knife makers (Who are starting to see high volume; as in, by the time one order is filled, he/she moves directly on to the next order, for a certain period of time.) should have a 1 knife per customer rule.