Reverb offers safe shipping insurance for when the gorillas at UPS/FedEx mangle your amp. Seller doesn't have to worry about refunds and buyer is reimbursed for damages. I've had to use it a couple of times and there was no hassle. Got reimbursed the next day. Without that protection I'd have been out several thousand dollars with a busted amp and no recourse from a "friends and family" payment.
I've also dealt with a few shit sellers trying to pull a fast one on a buyer by not disclosing details even when asked. Reverb protection saved me on that too otherwise I'd have been SoL.
Edit: I almost forgot to mention the dumbass sellers who put zero effort into packing up the items so it's 100% guaranteed to get destroyed in shipping. Reynolds wrap and 3 packing peanuts as the whole of protection isn't going to cut it. I get some protection from that fucktardery from Reverb.
That said I'd still do a purchase from a fellow active forum member that has a good reputation. Or if I felt okay enough eating the price if something did go wrong. But a large dollar purchase from a random dude on a forum, I want a safety net that's more than "trust me".