That was the case with every online store selling every conceivable product, it was not something exclusive to Gunbroker.
Ultimately, the day of the brick and mortar store is coming to a close, you can see that clear as day with the abject collapse of shopping malls. Personally speaking, unless it's something I had to have TODAY, I'm more likely to purchase something online and have it delivered even if it is exactly the same price in the store, not worth the hassle of getting in the car, driving across town, finding what I want, checking out, and driving home, wasting an hour or more, when I can accomplish the same thing in 2 minutes sitting at my computer. Hell, I'd even pay a little bit more for something online if it is less of a hassle.
Anyone who thinks the sales tax situation "levels the playing field" is missing the big picture. Even with sales tax included both ways, it's almost always cheaper to buy online, and a hell of a lot more convenient. Personally one thing the sales tax imposition has caused for me is to second guess if I REALLY want/need something. I always calculate that into the final price, and sometimes when all is said and done it just isn't worth it (it can turn a good deal into a not-so-good deal esp on ebay and gunbroker). The ironic thing at the end of the day is that for the states this likely ends up being a near zero sum game, sure they get more money from sales tax, but they are probably losing an equivalent amount in income tax from the seller due to decreased sales.
Sadly, many brick and mortar stores have patently refused to come into 1998 and have even a minimal web presence. Most of the gun shops in my area barely have a functional rudimentary website, let alone any online sales. These stores should have ALL been taking advantage of gunbroker for the past 2 DECADES, yet apparently they prefer to sit on inventory for years and sometimes DECADES when it's something that would sell in less than a week online for over asking price. There are probably at least a dozen gun shops within an hour of me, only one of them does Gunbroker, and only one of them has any sort of online ordering. And these are shops of all shapes and sizes (nothing quite so large it is a regional draw, but large enough to be a county draw), some quite small, some with $1m+ in inventory.