Shame, indeed. Surely it's easy enough just to put the price you want as a seller, which includes the postage to get the item to the buyer and any fees which may be incurred for the means by which the buyer pays. If the buyer sends cash, well, so be it ... sends as a gift by PP, so be it ... makes a bank transfer, so be it. The price stated should be the price that the buyer pays - no further amounts required.
If something goes wrong in between, well, as adults we should be able to resolve that. The forum and its moderators are not involved in the sale and should not be called upon to mediate.
Likewise, the method of payment should be agreed between the buyer and seller in private. Paypal does not suit some folks. Bank transfer may be preferred, cash might well be, meet up and hand over maybe. Again, adults really should be able to figure this out for themselves.
As for recourse when things go wrong, well, that is down to the buyer and the seller. Yes, the buyer might well be peeved if the seller simply says it's not his fault, but (again) as adults, we really ought to be able to sort this out. Any decent seller would refund and deal with the matter himself with the courier. I would think.
Furthermore, if the item is not as expected, well (again) that is between buyer and seller. I sold a razor recently that I'd actually never put together. I thought it was good, sold it in good faith but upon arrival it simply did not tighten up and hold a blade correctly. I asked the buyer how he wanted it handled (refund or return for maintenance) and agreed to take the item back to have a look myself. I refunded his return postage, cleaned out the crap from the threads, polished it up and send it back ... offered the full refund again, since I reckoned it would now sell for more, but he wanted the razor
Adults really should be able to figure this out for themselves ...
I really don't like the idea of rules. You're getting the idea that I tend towards folks making deals between themselves. I rather like the banter that goes on in the threads, folks popping up to give a little bump or to comment on how generous the price is or say how nice the item is ... clearly, folks are also doing the reverse which is not cool at all. That sort of thing should be absolutely private and by PM. The item, its condition and the price is the prerogative of the seller. If it's a crusty piece of overpriced rubbish, well, it won't sell ... simple. If there is something glaringly wrong with the item and that is not specified, well, take it up privately.
If there genuinely are folks taking the piss or simply being gits about sales going bad, well, there is the name and shame thread ... but again, dirty laundry is always nasty to look at and folks to decide if that is what they want to do.
I hope the BST comes back. I know I like to sell items here. I like to buy items here. I think it is a really good focal point for the folks here to try, move on, pass around, buy, sell, give away - it's good.