Some, like Quality Marine or ORA require that you have a physical store-front (not just a web-page) to buy from them. How they check this, I dont know (photoshop anyone?), but that seems to be their deal.
Otherwise, the bare minimum is that you have a business liscence, as well as a minimum balance amount. Then they will send you a list, and then based on that list, you order what you want. The challenge most of the time is meeting the minimum order amount ($500-$1000), and then keeping it. The other challenge is that sure, you can get some stuff dirt cheap, but the flip side of that is that if you order '6 anthias' or '12 zoa rocks', you will very likely get all males or all the same zoa rocks.
Lets just put it this way, if you are under the misconception that you can simply call up a wholeseller and order 'one of these, about 4" would be good, oh, and 6 of those, all different colors, etc'... you are in for a shock. If you think you can 'cherry pick or ask for pictures or something, you are wrong. Wholesellers are there to move merchandise, not cater to every little interest you have as a hobbyist. You have to think like you are a store when you order.
There is also the importer option (bypassing the wholeseller even). The hitch here is that sure, almost every fish you want is only $1-3, but shipping is where the costs come in. You often have to have a minimum order of 16 boxes (hundreds of items), and there is no absolute that they are all going to come in alive even (its these losses that produce the higher costs for the fish at the wholeseller level).
Anyways, I think you will find that there is a very good reason for why fish cost what they do. Often the best deal is to find a reputable LFS that has reasonable prices, and ask if you can get in with them on their orders, and just buy from them. Often, they will offer you a discount for ordering (and making sure to pick up what you order right away). They dont have to sit on the merchandise (worry about selling/keeping/feeding it). Often times, if the store knows you will be ordering alot like this, they will sell to you at a minimal upcharge from the wholesellers cost. IMO, thats really the best deal in most cases. Also, if it dies in transport, its not your problem.