sdtom: If I use your example - he would be paying income taxes if he took all his payments via Paypal. If you have $20K or more and 200 individual payments per year then Paypal will issue a 1099-K and report it to the feds. Part of the $20K plus also includes all the shipping fees charged to the buyers too. Paypal has everyone's SS numbers on file.
Odd enough - I have never ever paid taxes on a purchase at stamp show, nor on any auction items or most stamp stores except one. Maryland Stamps & Coins in Bethesda, MD has been the only philatelic venue I've ever paid taxes as a buyer. No, I don't have a tax number either.
Your example also has other issues is it's not solely about price.
A dealer is not just selling an item and nor is the price the bottom line. It's also about SERVICE. Better service and a better buying experience can bring in buyers and more important repeat buyers. Most of below would primarily apply to the variety of
ebay sellers - dealers or the occasional sales from a collector trying to unload dups.
Location - some buyers feel better about buying locally rather than from another country - cheaper shipping, speed of delivery and mail safety concerns. This is not a major concern, but it's there and of course some sellers limit where they sell to.
Knowledge of the seller can be a factor and larger dealers don't have a monopoly on any particular philatelic knowledge. A smaller dealer could be better able to sell a particular type of stamps or covers if they have more experience with material than larger dealer that doesn't and vice versa. Size doesn't matter in this case, but experience and knowledge helps to promote sales especially if it can be used to create better listings and able to share that knowledge with the buyer.
Quality of listings count - not just the actual quality of the material. A seller needs good photos and great descriptions to be found in the sea of millions of other listings. Too many dealers big and small skimp on this aspect too much and wonder why their sales are poor. If two sellers have like items, a better described item will be noticed a lot more regardless of price.
Scale of listings is a factor - the more you have to offer, the more buyers you will find. Larger dealers might have an edge here due to higher volumes of material, but it only counts if they can list it in an efficient manner. The more material listed the more chances you have to have a buyer come and look at your other listings especially since buyers like to combine shipments.
Quality and uniqueness of material - Dealers with deeper pockets might win some sales over another dealer who doesn't, but finding the right material is matter of lot of factors more than just having the capital. Lot of times I find just being at the right time and place can help me land some great material for very little. Sometimes it's just dumb luck or seeing things other dealers don't. Unique items help to bring in buyers for sure if you are the only seller that has it and so does better quality items (not necessarily always high dollar items).
I'm sure there's other factors to this multifaceted issue, but price is only one aspect in which dealers compete. You have to adjust for service, experience, location, knowledge, volume of material, quality and uniqueness.
Edit - Also if any online sellers were required to collect taxes for every state and municipality, and it has been proposed, the online stamp business would virtually dry up overnight. It's a logistically nightmare and undue burden. Also there's no reason to have a business license to sell stamps online. Half and sometimes more of what I sell goes overseas - can't legally tax exports.