The higher the grades, the more likely it is to be dealers. The lower grade bulk goes on eBay.
Lotting is a difficult one. You obviously can't combine lots from different vendors, so a single person's submission will really depend on the overall value of the consignment. You don't want lots with an estimate of a fiver, otherwise you would be all day making not a lot of money. That can easily lead to diverse lots.
I still don't see how any one person can cherry pick at lower prices in a free and fair auction. If somebody wants a lot then they will continue bidding, and if a collector, they can pay more than a dealer because the latter needs to make a margin.
Virtually every collector I have known has pushed the boat out on a bulk lot to get a particular coin of interest. Whether it is a bulk lot or a complete collection you are buying, in my experience the initial outlay is nearly always covered with interest when the unwanteds are sold, so on balance worth pursuing.