Often on eBay is sometimes you will find people 
who are willing to pay more than what something 
may actually be worth in any other market.  As 
someone once said about auctions, something is 
worth what two people will pay for it ^_^;;  If 
the bidders are completly informed about the item 
and still want to pay what seems to you a very 
high price, you shouldn't feel bad.  It is there 
decision, after all.  And you definetly can hold 
the 2nd highest bidder to their high bid, even if 
the 1st bidder does flake out, though you can't 
actually force them to buy it, as they didn't win 
the auction and therefore are not commited to the 
item anymore.  But it doesn't hurt to try.  
Basically, just let the auction run its course.  
It's not your responsibility to decide if someone 
is bidding to much anyway, just that they have 
all the info to make the decision themselves.