Living in Germany, I am affected on the other side of the transaction. I used ebay frequently in the past to get parts for my Landies. In fact it was the main reason why I signed in on ebay more than 20 years ago. I once got sent a complete radiator and front panel over from a nice ebay seller in the UK.
Even before Brexit, however, for short of a year maybe, it hasn't been what it used to be. It started when ebay introducted (or more and more sellers started to use) their silly global shipping programme, which massively inflated postage and 'handling' charges. There's simply no fun in winning e.g. an auction for a dash light for 13 quid and then spending another 13 or even more on postage. These days, even a simple gasket or oil filter often will only be shipped for a high two figure sum.
Now post-Brexit things have got worse with adding taxes even to second hand or used goods. A quite reasonably priced part can thus be so expensive until it arrives at my door, that it's economically just not feasible. I also relied quite heavily on professional UK parts suppliers and again often enjoyed transactions because of their quality stock. But the 'fun' is out of that, too. Due to shipping and, more importantly, taxes, the cost is very often (too) painfully high.
This is really sad, as for me, obviously, I need to turn to the UK most of the time if I need a certain part or want good quality. Additionally, to be quite honest, it was always also part of the pleasure and fun of owning an affordable classic to me that parts were plenty and usually not overpriced.
Sometimes I am lucky and find a part here on the forum or from a private seller (which is difficult outside ebay) and I always enjoy the contact with the friendly and helpful seller(s) greatly. Ebay used to be like that, too. I had made it part of my daily bedtime routine to browse the newly listed used parts on ebay.uk for many years, now I can hardly be bothered to look once a week.
In addition by the way, couriers have become lousy, too. Over the past 12 months I have lost at least 200 quid in goods which never made it to my door. The sellers could show me proof of shipping, but on their way to me they simply vanished. And I, as the person who bought the part and paid for postage can't file a complaint as this can only be done by the sender. Most of the times, they are willing to do that once, but of course you can't expect them to keep bothering postal services for weeks trying to track a lost parcel.
Geoff, if you are about to sell things, let me know