To encourage offers/sales, include a price.
This of course tips your hat as to how much you'd accept, so don't expect to sell for more than your offer price. (ie: If you have a domain that would go for $30 to a reseller but potentially $3,000 to an end user... I'd probably leave "make-an-offer", but many would disagree with me).
Putting ridiculous prices will only drive away offers,fwiw.
-Allan :gl: