If you don't want to set up another eBay account using the American version of eBay, what you could do whilst you are in the States and want to buy from a USA-based seller would be to send the seller a question using the Contact Seller option, saying something along the lines of "Although my account is registered in the UK, I am currently living in the USA and was wondering whether you would be willing to post the item to the address that I am staying at whilst in America." If the seller agrees to do so then add a secondary address to your existing account so that when you go through checkout you just switch from your primary address (ie: the UK address) and select your address in the States as the address that you want the seller to post the item to.
In the event that the seller does not agree to your request it may be necessary to set up a new account using the American version of eBay for the purposes of purchasing the item, even if you subsequently close down the American account upon your return to the UK.
Finally, if neither of the above options work, if you know somebody in the States who has an American eBay account ask him/her if he/she would be willing to purchase the item on your behalf. If the person you have asked agrees to do this for you then just give him/her the necessary amount of money with which to fund the purchase of the item that you wish to buy, then once the item has been received he/she can then hand it over to you.
