Container homes can be placed on a variety of surfaces because the steel box is self-supporting. For example: “Container homes can be placed on a wide range of surfaces. Container homes are self-supporting and do not need minimal foundations.”
However, you’ll still need a level, stable foundation or pad (concrete piers/slab/footings) depending on local soil/climate. Also utilities (water, sewage, electrical) must be connected.
Some providers recommend concrete piers plus slab or crawl space.

Your container home can be expected to last at least 75 years. Another indicates “at least 25 years … or longer if well maintained or with siding.
Rust, moisture, insulation failures are main risks. Regular maintenance helps extend life.

Yes … there is a market for pre-owned units.
Resale value depends on condition, location, finishes, zoning acceptability. Maintenance matters: rust, structural modifications, permits all impact value.
You should provide guidance on inspection and certification if buying used units (to ensure safety, no toxic coatings, structural integrity).

This depends on local regulations. Some jurisdictions permit container homes as permanent residences if they meet structural / safety / insulation standards. One provider states that because their homes are code-compliant, financing and permits are possible.
You should make clear: Buyer is responsible for local zoning/permit compliance and you (seller) will provide documentation/specifications to assist.
Also mention shipping containers may be treated differently from “traditional homes” — you may need to check whether your area accepts them.