Its possibly just surface rust. However to check you're going to have to do the squeeze test between fingers and thumb on the thicker structural sections, then if you still can't determine how severe or not the rust is.. Its time to get an old screwdriver out and tap (with the handle part) on the rustiest bits.
If the material is corroded right through holes will likely appear, this is not good especially in the structural sections,if this is the case then you may like to walk away from this one as welding will be required. If however it is just surface rust then a scrap off and clean up followed by an application of underbody protection will suffice and your good to go.
The seller should have no issues at all with you taping on the structural parts of the body to check its integrity. You need to know the vehicle is solid, & safe.