I make offers all the time, it's the only way to buy. Mainly on cars and more expensive stuff and only when it's local. Send them a message, go look at the item and while you're there sus out what they want and make an offer to take it away right now. A lot of sellers are decent people and car enthusiasts and are very likely to take your offer if they've had a good chat and realise you're an enthusiast too. The last think a genuine seller wants to see is his item listed on ebay the next week for double what he sold it for, so he'll often take a bit less if he knows you're not out to make a quick buck.

If the seller is a tool then you weren't likely to get it for a decent price anyway so you haven't really missed out on anything.