the UK import duties are not including in the GSP, it states that in their terms, but if you request an invoice from the US seller he has the option to opt out of the GSP, i just message the seller before i bid and everyone so far has agreed to opt out.
ebay auto opt them in, its a total scam to make money, paying ebay to ship your item for you at 3 x th cost.
"Import charges": this Programme Fee consists of charges associated with the customs service described above and is comprised of the following variable amounts: sales, goods and services, and value added taxes, duties, tariffs, excise taxes, and other amounts assessed or levied by any government authority in connection with the importation of goods into the applicable country of importation (but excluding income taxes) ("Commodity taxes"); third party brokerage fees (including advancement and disbursement charges and customs brokers handling and filing fees); penalties (but excluding any customs duties, taxes, surcharges, fines, penalties, or other charges which may be imposed on you by customs or tax officials after a GSP Item has successfully cleared customs and been delivered to, or made available for pickup at, the delivery address that you specify); classification charges associated with the assignment of a Harmonized System ("HS") classification code; charges for export compliance screening and verification and the assignment of an Export Control Classification Number (ECCN); and operational expenses associated with short-term loss recovery and the management of variances between the quoted Import charges and actual costs. Note: at eBay's discretion, applicable classification and export compliance charges, third party brokerage fees, and operational expenses may instead be included within the Shipping amount that is quoted to you.