Gosh cheques, yes I remember the days when it was postal order, cash in the post or cheques to pay for eBay purchases! I don't think I ever used postal orders, but I certainly used many a cheque and sent cash, generally bundles of dollars over to the States.
I bought A LOT from the US back before the current extortionate postal service system came in on eBay where they add about £18 postage onto your order plus taxes. I got a lot of my early days silver from the US.
Fakes were always a risk but now they seem as common as real coins.
I've been buying on eBay since 2003 and most of my issues have been in the last few years, none of which were PM related.
I stick to sellers I've used before, with good feedback. If buying from a new seller, I check their other sale items to gauge quality or issues. Then I check their feedback. I also only buy if the photos are clear enough to be certain that the item is genuine (assuming the photo hasn't been stolen from elsewhere). I do as much as I can to protect myself.
I sold PMs on eBay from about 2012-2018 (mostly lots of junk silver but also sovereigns and half sovereigns). I never had any problems.
I never posted abroad.
I always included clear photos of obverses and reverses and also a photo of the coin/s on a set of scales.
In group lots, I'd list all the dates individually. I would also give clear descriptions of the coins, drawing attention to any problems, cleaned, holed, polished. In fact I made more of a fuss about the defects than anything else. If you make it clear from the off, the buyer doesn't have a leg to stand on complaining about a clearly listed and photographed defect.
I also nearly always had a £1 starting price with no reserve and left it to the whims of the Gods to see where the price ended up. I didn't want to pay for Buy It Now.