Not if its done through an instant transfer or your card im afraid not... if its an echeque you could cancel it.
It might be worth ring paypal and asking them? However i think its a binding contract or something when you buy things online .i know its not through ebay but heres their terms-
Distance Selling Regulations
The Consumer Protection (Distance Selling) Regulations 2000 generally apply to sales to consumers made by sellers acting in the course of a business, which have been made at a distance. In other words, where there's no face-to-face contact between the seller and the consumer before the contract is made. The Distance Selling Regulations apply to items purchased via Buy It Now listings and Second Chance Offers on eBay.co.uk. However, they don't apply to auction format listings on eBay.co.uk.
The Regulations also provide a period of seven working days after the date of receipt within which the consumer can cancel the contract (often referred to as the "cooling off" period) and get their money back, including the original postage and packing charges. The consumer can be asked to return the goods at their own expense, but only if the seller informed them of this requirement before the contract was made (otherwise, the seller is responsible for collecting the goods). If a business seller has not provided the information required under the Distance Selling Regulations (as discussed above), the buyer will have up to 3 months to cancel the contract and get their money back.
The cancellation rights do not apply to certain items including software, audio or video recordings that have been unsealed, perishable goods such as food or flowers, or items that have been made to order.
