Implications of brexit for UK-EU personal purchases and sales

Until December 2020, I was happily in a customs union with a lot of other countries.

If I needed to buy a hard disk for my computer, it was often cheaper (including shipping) or more likely available in specific models, to buy from the Netherlands or Germany than the UK. The few times I upgraded my PC or other things, and had old computer parts spare that I didn’t need any more, to sell, one went to Spain, the other to Greece.

None of these were trade, and none were items bought in order to sell. They were all occasional bona-fide personal purchases, and disposal of used personal possessions 2nd hand. Total value might be £2000 or something, each way, in a year. Could vary in future. Say 70% eBay (purchase and occasional sale), 20% Amazon (purchase only) and 10% individual vendors (purchase only), if that gives an idea. Assume for this question that it’s self evidently not trade.

Now that we are post-brexit, what has changed? What do I need to know next time I buy from or sell to an EU country?

I’m thinking broadly here, because I can imagine everything from forms and declarations, to different courier procedures, taxes and duties and how and who they are paid to. Ditto for whatever I need to make sure a seller does, or a buyer knows to expect. All the things I take for granted until now.

What do I need to know/do?