You've been lucky. I'd say 1/3 of my card sales over the past year have ended up being returns -- it seems to have gotten worse over the past few years as "buyers" look at it as a rent-to-try service with many gaming the system. During high-volume/high-liquidity/high-priced times like this around GPU launches, the probability of you getting a return are much higher. And like you say, buyers can return for any reason whatsoever for 30 days with no recourse for the seller and the buyer always wins. In short, it's asking for trouble selling now.
As an aside, while there are other risks with this in the US, it's much less likely for sellers to get burnt as the return period is much shorter and the seller is often protected. When I moved over to the UK, I was super surprised how many people felt entitled to return goods that they'd effectively contracted to buy that were perfectly good and as described.
The service is a long way off when I first used it in 2000 to buy a Pani Laser Disc player and lot of 30 Criterion discs for $60. The seller was local, asked me to stop over to collect, and had me stay for a lovely dinner with his family (which I kindly accepted being a destitude student).
Edit: My apologies, missed responding to your question on guidance. A few points I follow:
- Sell at a reasonable price
- You can't add hard rules, but I have disclaimers about not selling to >1-year members with less than 20 feedback
- I have a few other disclaimer points to scare away fraudsters e.g. only sending to confirmed address, shipping promptly (I've had a buyer ask me to wait to send for a week, obviously trying to time to have return window end after a card launch), taking serial numbers
- Never sell within 30 days of a card launch, or when stock of new cards is incoming (now, for example); state no low-ball or out-of-terms offers
- Sell at a reasonable price, making the sale more attractive to keep for the buyer -- don't be greedy. I sold my RTX 4090 Strix in late November for £1650 which was a fair used price, and not one that the buyer would be looking to return now. Remember those £2000+ sold prices you're seeing at present won't stick have a super high probability of the buyer returning and/or other tomfoolery -- not worth the headache
- Communicate with the buyer. You can often suss out bad apples, and if you're spidey sense it tingling, don't accept offers from sellers (if offers are in play)