I use GB a lot with good results. However- no matter how good the deal I won't purchase unless the seller accepts a credit card, preferably using GB checkout. At least lack of fulfillment can be disputed with the CC provider.
Yeah, some folks have the money available to do that - Uncle Sam being a great example. I worked for a federal agency for a few years back in the 1990's, that had an official travel policy: they wouldn't purchase airline tickets unless they were refundable (in case the employee got sick, or whatever). Sounded ok to me, until I was lining up tickets for one work trip, and discovered that I had two options: $200 for a non-refundable ticket with a budget airline, or $400 for refundable with one of the majors.
I went to the travel coordinator and pointed out the obvious: it's brain-dead stupid to waste $200 right up front, just so you MIGHT be able to avoid wasting $200 if some unplanned event occurred. Didn't matter - Uncle Sam insisted on pissing away the extra $200 for 'insurance' on a $200 ticket.
And I see the same thing happen on GunBroker - a few months back, I watched a guy pay $40 for a Colt stock, when I had an identical stock listed for $20 (but no option for CC payment with me, because I'm not a business). Funny thing was, I had straight A+ feedback for over 20 years, which was better than the other seller. Didn't really matter to me (somebody else bought my stock), but it always makes me wonder!
Like a lot of things in life, I guess, you make your choices and pay your bills!
As always IMHO, FWIW, YMMV, etc.