If you don't plan to keep something in your collection, you should not accept it at all.
You can accept something as a "non-accessioned" gift, with the intent to sell it later, as long as you are up front with the donor so there is no confusion later about the museum "selling off the collection." You don't want to give the community the wrong impression. If you have a reputation for selling items from your collection, then you will find people are unwilling to donate to you.
This gives me an idea for an article! Next week we will take a look at the deaccessioning process. Thanks!