I was just on GunBroker after having read this thread.
I do like GB, and have bought a few guns there and been pleased with the process.
What I went specifically looking for were some of the search features mentioned. I did not find the advanced features, and I did look into what they call their "smart search". I found a few drop down menus and the regular text search. So if you are looking for a 10mm you have to type "10 mm" into the search box. Then you have to do it again for "10mm" and you might still wonder if some doofus listed his under "Ten Millimeter" or "Ten mm". Typos on mfr names, model names, hyphen included or excluded? Oh, you're only interested in the rare 4" version? Good luck with your search. Better put on another pot of coffee.
Guns are not terribly complicated or diverse when it comes down to it, no more so than dogs. Dogs, whatever breed or size, have 4 legs, a long tongue, eat dog food, etc. Guns all have barrels, shoot ammo, etc.
I'd like to see their "smart search" improved to let the searcher discriminate between mfr, type (handgun, shotgun, rifle) sub types (like revolver/semi or pump/break-open, etc), by caliber, by barrel length, by color/finish/material (such as blued or stainless). I've used the Davidson's search engine and it incorporates many or most of these features. If it allows multi-select, even better - say you are looking for a 9mm or a .40 and are willing to take blued or stainless but not parkerized, and you're ok with alloy frame or steel frame but no plastic... Oh and you're only interested in guns within your home state so you can drive there and inspect it before purchase. Now, that kind of specific search might give you a reasonably short and easy to manage list, instead of 150 bogus leads that you have to sort thru manually.
There's no reason a search engine like this couldn't be made, and it would probably be very popular with buyers. Sellers, on the other hand, might not like having to click on a few additional boxes to list the item. To me, the benefits would outweigh the costs. Oh, and you should be able to set an "alert" of some kind when a gun you have been looking for becomes available, for instance a da-sa trigger with a parkerized frame in .380 - not going to find one every day and if you're not doing a search every day you might miss a rare opportunity.
Then again, maybe some or all of these features exist and I am just ignorant on how to access them.