The biggest second hand online marketplace in Finland did a big update to their service recently. In addition to overhauling their whole user interface they also added a few new features.
One of the biggest changes was the ability to take offers for the items you are selling. As a buyer you can make an offer and the seller has 24 hours to wither accept or reject the offer. It's a bit like an auction but not really. As a buyer you can't see what others have offered so there is no way telling whether your offer will be valid or not.
Without a set price or seeing what others have offered makes it difficult to make an offer. It doesn't really encourage me to offer anything. If I offer too low I most likely miss the opportunity when someone else offers more. I don't have the possibility to make a counter offer. If I really would like to win the bidding I would need to make a relatively high offer, potentially paying overprice.
The system doesn't favour the seller either. If you would want to get the best price you would need to be quite active looking for offers and accepting the right one at the right time. As the offers expire after one day you might lose a good offer in hopes of getting even better one.