This is exactly what craig's list is for.

For trade: Girlfriend. Condition: Used*, somewhat stupid, but fully operational. Plays video games!

Will trade for a back cover for Nokia N900.

* Used: items that have been used before. This item may have some signs of cosmetic wear, but is fully operational and functions as intended. Comes with no warranty, except for in case of DOA. Seller will not cover any incidental damages to Nokia phones caused by girlfriend.