Thanks for the comments.
I understand you, and many sellers, would prefer to put ALL of the Risk on the seller. How nice for you.
But there a few problems...
1. All functional (and most cosmetic) issues can be hidden with good photography.
2. AG advertisements require an item to be assigned a condition-rating (e.g. 9/10) and to state any defects (both functional and cosmetic). If it ARRIVES (to the buyer) in a lesser condition, you are in breach of that contract (i.e. false advertising).
3. Cash up-front (including F&F), with no item in-hand leaves the buyer stranded, hoping they are not getting ripped-off. If an issue arises, the Buyer does all of the work to rectify the problem - Truly unscrupulous.
4. Sellers may have gotten the items from the gray-market, with absolutely no provenance, with the intent to 'flip' them for profit. Consequently, the internal parts may not be original parts or may not even be functional.
5. The items being sold may be a knock-off items. This is a very common issue with cables, tubes, etc. - it seems that China is especially adept at imitating trademarked goods.
If I were working with a Used Car Dealer, instead buying your used audio gear, at least I would have the chance to drive it, look under the hood, kick the tires, etc. BEFORE turning over my cash.
Clearly, your sales process falls below that considerably very-low bar.
** Again, I encourage ALL BUYERS to avoid buying from dealers who will not stand behind the sale. It is our money, and we deserve to be protected from scammers.
Ethical sellers should know enough to avoid that crowd by using a buyer-protection backed medium (ex. Credit Cards, PP, etc.).