Is this a theoretical question or do you have a specific listing in mind which has you perplexed?
🙂 Some things can be changed on a listing, but it depends whether or not there are bids, or how much time is left on the auction. If it is a specific item you have seen, the seller may have deleted and relisted, - or with no bids sellers can change the price, drop the 'buy', lower the reserve and so on. The following link will show you all the things that can and cannot be changed, and under what circumstances:
http://pages.ebay.ca/help/sell/revising_restrictions.html