Base your offer on what YOU are willing and able to pay rather than on what the Buy It Now price by that seller is. You have no way of knowing how much over his/her true maximum, if any, s/he set the BIN, or how willing s/he might be to haggle (which is very likely not at all since eBay tends to put Best Offer on listings for seller who don't want it).
If the BIN is within your true maximum seriously consider taking it rather than making an offer. Aside from the very real probability that the seller had no desire to have the BO option, someone else might make a higher offer or take the BIN while your offer is pending. Bid, hand, etc.
