You can certainly ask the tenant if they want to buy it if you like--and I think that would make good business sense because the more potential buyers you have the better offers you should receive--but I wouldn't do it out of any kind of implied obligation. They have paid rent. You have let them live there. You have both lived up to your respective ends of the deal.
I do agree with making it a "win win" for the tenant regardless though. You will need their cooperation to have good showings to attract the most buyers.
Curious, is this an "investor only" house, or could it be an owner occupied?
If it's an owner-occ hood, then definitely consider cash for keys to get the tenants moved out early. Most retail home buyers won't be interested in a property where a tenant will be present and possibly tearing up their soon-to-be family home.
If it's a home only an investor would want, then they'll want to see that the place isn't a disaster, but they won't cause too much disturbance if you set reasonable showing expectations. You might offer your tenant a couple hundred $$$ per month to have "open showings" each week between 5 - 7 PM on Mondays and Wednesdays, for example, as long as they keep the property reasonably clean and picked up. Not "Home and Garden" TV immaculate, just not a lot of junk strewn all over.