If he's got the "make offer" option, making an offer is fine. Some sellers like their stuff a bit too much and get cranky with lowball offers, but I'd think 10-15% less is reasonable.
I think offers on BIN items without the make an offer feature turned on are maybe a bit of a gray area, but I don't think it's that far over the line to ask.
Now making offers on current running auctions is totally different. Others feel differently. (And please lets not make this one of
those threads)
Steve B