@tintin
I get offers like that all the time and I turn them down. It is against eBay's TOS to end an auction after there has been a bid. Also eBay keeps track when someone does this. If you plan on doing business on eBay in the future I'd recomend not breaking their TOS. :-(
-Edit-
The people making these offers almost never bid on the item after their offer is refused anyhow. What does that tell you?