I've often wondered about this – eBay listings with ludicrous prices - so have looked it up.
Apparently, the reason is that eBay offers no option for sellers to list items as being temporarily out of stock. They can only end or take down the listing, which means the message stating “100 sold - over 10 remaining”, which is encouraging to buyers, will be lost for good.
If they take orders on an out of stock item, they risk being unable to fulfil orders if delivery is delayed/their supplier is out of stock, etc. That may cause them to fall foul of distance selling Reg’s and eBay’s rules and harm their feedback ratings.
As a result, a lot of sellers hedge their bets by putting the price up to something ludicrous whilst they wait to see if they can get more stock.
Doing this for a short time also helps keep their place in Best Match (so their item will be somewhere near the top of the list when people search) and retains the sold history. Doing it for longer is counter-productive, however, because every visit to the listing that doesn't result in a sale reduces the Best Match weighting.
Interesting, eh?