Same Stamp - Different Sites

I've lost out on two auctions on ebay this weekend for a stamp I really want. In disgust, I went to look on bidstart to see what they had to offer.
Looking through the listed items, I see a stamp that looks familiar. And sure enough I've seen it on ebay (it has a very distinctive cancel), both for sale/auction from the same seller. And, I may add, at different prices!
Is it smart or ethical to offer the same item on two different sites? What if it sells on both sites at the same time (a long shot, I know, but let's just pretend)?
As a seller, aren't you setting yourself up for possible problems? And as a buyer, what if two of us have bought the same item at the same time? Wouldn't you be p'd off? Who gets it?
Thoughts?

Last week, someone said that bidStart has an option (and an agreement with ebay) to list the same item on both sites. I can't find that post now. I recall that the first buyer terminates the sale on both websites, so there's no disputes.

James bidstart allows sellers to sync there ebay auctions.
If the item sells on ebay first the bidstart listing will cancel and vice versa.

Thanks much for the responses. Does seem strange to me that his price and shipping rates are different from one site to the next. But I guess that's what shopping around is for.

From the BidStart info pages:


Regarding different prices..., I assume you are free to edit your listing once they get synced.

It makes perfect sense to me: ebay takes a much bigger cut than Bidstart does, depending on the item in question. Also, Bidstart allows sellers to accept those evil, Satanic things called "checks" and "money orders" in which case there's no PayPal cut of the item & shipping costs.

That said, I personally have found the sync system between ebay and Bidstart to be a bit wonky. If you change the price on one site it doesn't necessarily change on the other, so I can see how it would be easy to miss a modification.