Welcome to the forum.
Sellers of surface land often do want to keep the mineral rights. They would have to have the proper wording in the sales deed to sever the minerals and retain them. Mineral rights are generally dominant over surface rights, but there are some cases in Texas that are trying to change that for solar farms. The sales price is negotiable and can reflect not including the minerals. Yes, you can sell the land later without the minerals. A title search through the county courthouse would record earlier sales and see if a severance was already listed.