Always ask for the stuff to be sent via United States Postal Service (USPS) and avoid UPS, DHL and the like. If your item is below £19 in value then no VAT is due. If your item is above this, then it MAY be caught by customs and VAT imposed. If the parcel does get caught the UK Post Office charge an £8 fee for collecting the duty.
If your parcel comes via one of the commercial parcel expeditors like UPS then you ALWAYS get caught for customs duty (VAT) and their fee, which is higher than the Post Office fee.
I've bought quite a lot from the US, both eBay and others and mainly the stuff gets through without extra charge.