Indeed, if you look at completed auctions and can see the item hasn't sold several times over then messaging them with an offer may be worthwhile. If this is the first time the item has been listed then the seller has no information how much interest there is until the closing minutes.
(this presupposes that sellers know how ebay works - it's always possible that the seller doesn't and wasn't aware of settings like 'allow offers'. There's sadly an endless supply of numpties on ebay, both buyers and sellers)
Theo