Don't want to stray too far off topic - but, "bidders premium"?
Yikes!!! This is ridiculous. Is it flat rate or % of the winning bid amount? GB charges a couple of bucks a year!
It's good old capitalism; it's how the auction house makes money. Some charge both the consignor and the buyer a percentage (typically lower for the consignor), some only the buyer (consignor percentage depends on the auction house and the size/value of your collection). The bidders premium is a fixed percentage, so $300 bid with a 20% premium is $360, $500 bid is $600 etc.
If you're going to bid at these online auctions through Proxibid, iCollector or other online intermediary services that list offerings from physical auction houses, or directly with the auction house where offered, like Rock Island Auctions, then you need to know the ins and outs. Proxibid of course adds about 3% to make their money, so if you can bid directly with the auction house, and if you can pay cash (check, money order), you can save quite a bit of money, and/or afford to bid higher. Understand the bidders premium percentage, and read through the "terms of sale" tab to see if there is a CC fee (which may or may not be part of the bidders premium), an "FFL fee", a "packaging fee" or any other such adder.
My approach is to determine the maximum I'm willing to pay for an item, then I subtract any flat fees like an FFL or packaging fee, then I divide by the sum of the bidders premium and CC percentage. For example, if there are no extraneous fees but the bidders premium includes the CC fee and is equal to 20%, and the max I'm willing to pay is $600, I divide that by 1.20 = $500 for my maximum bid. I always prepare a spread sheet using the bid increments found in the auction houses increment table and multiply them by the percentage to get the true "cost" of any "bid".
GunBroker has little overhead and relatively few employees, so their fees are relatively low. It is definitely not "a couple of bucks per year". Their fee is 6% of the value up to $250 and 3.5% of the value beyond $250. They don't run an onsite physical auction house, they don't handle the firearms, they don't package or ship the firearms, they are a
no touch facilitator that merely provides an electronic forum for sales. They're more analogous to Proxibid, but their fees are actually higher relative to Proxibid. They are obviously lower than the fees of Proxibid plus the auction house.