The obvious...the lease has already been mentioned. If you are selling to an investor, having a lease in hand is a bonus. If you are selling in an area where an investor is not the likely target, your typical buyer will make this home their primary residence, then you will have other concerns to address with the tenant. There are a few cases where tenants are helpful and easy to work with, in most cases you will find them reluctant to allow showings, or they will have a lack in care for the property ie. it will be messy during the showing.

It would depend on how your lease is written. Hopefully your lease addresses the sale of the property. If the new owner is an investor this may be of benefit to him buying an occupied property with a long term lease. I would recommend speaking with a real estate attorney to see what options you have.

The question one needs to know is what kind of lease do your tenants have. If they have a lease, typically the lease runs with the sale of the property. The new owner will have to honor the lease. If they do not have a lease - typically written notice with proper documentation that they have received it is required. Contact an real estate attorney if you are concerned that your tenants won't cooperate.

Good luck!