I've only done 1 deal on ebay (I was the seller), and as soon as I recieved that email form ebay telling me the auction has finished, I contacted the winner straight away. I aren't that difficult, just write a few lines saying payment method, address, and I expect to hear back in a day or so too. I think there are only a few reason why the seller won't respond.
1 - They never had the item in the first place
2 - They changed their mind about selling.
3 - Their computer has died.
I don't think any excuse stand up these days, even if my PC dies, I can still get access in other PCs to access my emails. Though not the way I like to do it (through Outlook Express), at least I can stil keep contact with the outside world.