If it's not a legal bedroom (window with egress, a closet if that's required), don't advertise it as a bedroom. However--and, again, check with a lawyer--you probably couldn't stop someone from using a room in the basement as a bedroom if they so chose. Sure, you could write something into the lease, but I haven't seen that done.
One other question: Is spending $3,000 for the window to make the room a bedroom the best use of that $3,000? You might check with a Realtor. But, for instance, if the house is already a 4 bed/2.5 bath, it might not be the best use of the money. Maybe it could be better spent on landscaping, or new carpets, or paint, or a bathroom rehab. On the other hand, if the house currently only has 2 bedrooms, it could make sense. But you need to address that point, if you haven't already. And you need to ask whether $3,000 spent on making it another bedroom will result in an adequate return on investment (ROI). For instance, if you spend $3,000 and can rent the house for $83 more a month--or $1,000 a year--that's a 33% ROI. On the other hand, if the house will rent for only $10 more a month--or $120 a year--that's only a 4% ROI. There are also various tax implications; check with an accountant for details.
Hope that helps.
The price tag you put on the egress window caught my eyes, as well. I would recommend you check with several contractors about the cost. I have heard of egress windows being installed for less than half the price you quoted, which may make the option seem a little more palatable to you.
If you are a do-it-yourselfer, check the link below for directions on how to install an egress window yourself and save a small fortune.
If you are still trying to sell it maybe you could let your agent know that you would consider putting one in for a buyer. You don't just want to loose a buyer over a $3,000 fix in today's market. Your agent could let buyers agents or buyers know as they come through if that is a problem you are willing to fix it.
In regards to the legalities. I would really suggest that you discuss this with either an attorney or your insurance company. You might not want to risk it if you are having renters in there and you have knowledge they are using as a bedroom even though it is not legal because of the window. Might give you more peace of mind just to get it done.
I hope this helps! Best wishes to you!
I am aware of a real estate company in Michigan that was taken to court. One of their agents advertised a home as having a basement bedroom. After the sellers moved in they used the room as advertised. There was a fire and someone was badly hurt. Would you want to place yourself in the same situation? Remember, information on the internet, stays on the internet.