I don't know how to look up how long I've used eBay but it's been about as long as you have. I've sold dozens (maybe over 100) of items over the years. I've also used USAM for a few items but never Audiogon.
I've had good success on eBay over the years but the last thing I sold was returned for a reason I didn't feel was justified. Before that I've was scammed a couple of times but those cases didn't cost me any money. Nevertheless, I will keep using eBay for two reasons.
1) I sell all of my items as an auction with a low starting bid. Every item is guaranteed to sell whicy is a big factor for me. I have nearly always received a fair price and in many cases my gear sold for more than I thought it would. I don't like the process of listing an item on USAM and receiving lowball offers. When I put an item on eBay I know it's going to sell in a week and I'm willing to take whatever I get.
2) eBay has negotiated discounted shipping rates which makes the delivered cost of the item significantly lower to the buyer. The process is easy and seamless to get the item shipped.
I've got a couple of items to sell at this point and I'll see if my luck holds out. I post a lot of pictures and a lot of detail on the item I'm selling. I've had a couple of times where the buyer challenged me but the issue they had was clearly explained in the description. In other words, they didn't carefully read the description of what they were buying.
Your post is interesting because I share your frustration. My experience is that people are getting worse. They are more dishonest, more confrontational, more crass, and more unreasonable. Transacting business with average everyday people is getting more difficult all the time. I feel like I just can't trust people anymore. It's a real bummer.

