Sellers can end their listings at - almost - any time. Exceptions are if there is less than 12 hours to go on an auction style listing that has received a bid.

The seller may opt to no longer offer the item for sale for a variety of reasons (ran out of stock, the item was damaged or lost, no longer wish to sell, and so on). For most intents and purposes, an item listed on eBay is simply an advertisement, and an advertisement can be ended for whichever reason the seller chooses. I suppose the dodgiest reason would be an auction started low and not reaching a price the seller was happy with,which is not technically kocher, but even then there's not really much that can be done about it.