I agree with Cheryl that it is common in Texas for the seller to want to retain the mineral rights. It is a personal opinion of mine that when I buy a home, I want to obtain the mineral rights because I want the whole thing. Sometimes it is out of the sellers hands to offer the mineral rights because they have been retained by the person that they bought the house from. I also believe that the minerals are just a way to negotiate more money for their house. A lot of times, if a seller is saying they want to retain the mineral rights, they will negotiate for them. If they persist on keeping them, maybe they are worth some money, which is why the seller doesn't want to give them up. As far as resale value, I do not believe at all that it will affect the value of your property. As a matter of fact, I never base my valuations on whether or not the seller has the mineral rights, and also, I don't know that appraisers use this as a value factor either.
Hope this helps. :)