Any offer or counteroffer will have a stated expiration time (usually either 24 or 48 hours) but that is a maximum, not a guarantee that it will last that long. All pending offers or counteroffers immediately and automatically expire if the auction gets a bid or the listing ends for any reason (e.g. someone uses the Buy It Now or another offer or counteroffer is accepted or time runs out on the auction).
NOBODY (buyer or seller) is under any obligation to respond to an offer or counteroffer. S/he has the right to do nothing and let it expire.
As the buyer you are obligated to pay if the seller accepts before the offer expires, so unless you are prepared to wait out the full time you are quoted when you make the offer (up to 48 hours) do NOT make the offer and don't go shopping for the item on eBay or elsewhere except for amusement ("I found a better deal" is NOT a valid reason to get out of your obligations and promises). While the seller is also obligated to sell if you accept an offer or counteroffer from him/her s/he has a bit more leeway since s/he is entitled to take other actions in trying to get a completed sale on the listing (making other offers/counteroffers or even ending the listing manually) that would cause any offer to you to expire early.
