If it was on here that you read it, it was probably from me..
The Sale of Goods Act 19somethingorother clearly defines when a sales contract is in place.
Briefly, it says the sales contract is in place when buyer and seller enter into agreement on the terms of the sale.
There's also a part that states that it is not a legal sales contract if the terms of the sale are set by a third party.
It does cover sales by Auction Houses (I think the act does that and not another act) which does cover online auctions like the BBC Worldservice one but eBay IS NOT an Auction House.
eBay public states - mostly when being sued - that it does nothing more than provide a forum where buyers and sellers can meet. It provides a kind of price discovery that people can then use for a sale.
So, until you speak with the seller and actually agree via email or eBay message or telephone etc on the terms of the sale (including what payment method) then no sales contract is in place. Neither can enforce it until that point.
The enforced use of PayPal by eBay is against the Sales of Goods Act too - have seen posts on forums of people bringing this up with eBay after stating they wouldn't accept PayPal and having their listings pulled. Once the Sales of Goods Act was mentioned, eBay let the listing go ahead.
That's one reason I never pay straight away without trying to get some kind of communication, a sales contract, with the seller first.
If you could be bothered, after agreeing the sale via email and the seller then getting a better offer elsewhere, you could in theory take them to the small claims court to enforce the contract or receive damages for the contract being broken (cost of buying a more expensive one for example).
Have seen some walk through online of a case where a poor guy walked away from an eBay caravan sale because it was mis-described. The seller got all shirty about it and threatened to take him to court. And he did! Because the buyer didn't take the defence seriously enough and wrote a brief and shoddy one on the small claims form and then didn't turn up to court, the dodgy seller was awarded compensation for the breaking of the sales contract..