I believe from what I’ve read elsewhere, that the EU/UK relationship with regard to buying and selling anything - including used goods - will be the same as the current UK/rest of the world relationship. So you’ll pay VAT and import duty on anything you buy, as will anyone you sell to within the EU. We become a third country. The VAT rates vary across Europe, you currently pay the local rate - the rate operable in the country of purchase - from January you’ll pay the UK rate, which might be higher.
Buying from somewhere like Thomman will be the same, though you’ll probably end up paying import duty on top of the VAT. Couriers will probably charge a little extra too, I know Royal Mail add a handling fee onto purchases from America.