I have only used Gunbroker once. I used it when I bought my PPK/S. Everything went ok, but it seemed a little more complicated than eBay, but still pretty decent. Also, I found the whole 15 minute thing a little confusing.
Outside of that, Id use it again if there was something on there that I couldnt get locally that I HAD to have.
The 15 min rule is great for buyers and Sellers. Keeps the Ebay last second bid snipe from happening. Sold and bought on GunBroker. Fees are much more reasonable than Ebay and it gets a lot more visibility than some of the local sites. Never really liked guns America but that's just my personal opinion.
Bought and sold on GB, no problems at all and generally you get a better price if you are selling. Some good deals there, ask questions of the vendor and be sure what you are buying..I try to do business with folks that have a rating.