I used to buy a few things on ebay from the US but not any more because of the import tax and the fact that the carrier makes a charge for collecting it on behalf of HMRC.
Royal Mail charge £8, other carriers charge more.
You can appeal the tax if you have an invoice and HMRC are quite good about refunding it (if, for instance, there has been a mistake or omission on the Customs Declaration form) but getting the charge back from the carrier is difficult.
Goods below the value of £15 are exempt (whoopy do!!) from the import tax system and HMRC are supposed to waive the tax if it comes to a piddling amt - but you know with Customs Duty, Excise Duty and VAT the system is very hard to understand.
Honestly, I wouldn't even consider it now unless it was something I couldn't live without.