Don't sell your mineral rights...But, depending on what you want to use the land for, you may consider leasing the mineral rights for a nice initial payment AND a good ROYALTY on money made from what's extracted until the terms of the lease are over...Never do a deal for one lump-sum payment unless the terms are in actual dates...As others have mentioned, see an Attorney in that field...
I'd never to business with anyone who just called me out of the blue though...Be careful...