Can I sell properties in Monopoly with houses to another player ?
Unimproved properties, railroads and utilities (but not buildings) may be sold to any player as a private transaction for any amount the owner can get. However, no property can be sold to another player if buildings are standing on any properties of that colour-group. Any buildings so located must be sold back to the Bank before the owner can sell any property of that colour-group.
Houses and Hotels may be sold back to the Bank at any time for one-half the price paid for them. All houses on one colour-group may be sold at once, or they may be sold one house at a time (one hotel equals five houses), evenly, in reverse of the manner in which they were erected.

From:
http://richard_wilding.tripod.com/monorules.htm#sellingproperty
The following page is a copy of the rules as printed in the Monopoly® Rule Book.