World Order · Board game rules
Can I sell or spend Tanks from the Main Board?
Quick answer
No, Army tokens ("Tanks") aren't a tradeable or spendable resource — they can only be moved or used to Build a Base, not sold or spent from the board.
Details
Army tokens are military units placed on the Main Board through the Move and Build a Base actions. The Move action lets you take an Army token from your Player board or the Main Board and relocate it to a Region in your zone of interest or where you have a Military Base, paying 5 [money] per token moved [p.14 — Move (Military)]. The Build a Base action lets you move Army tokens from your Player board to a Region as part of establishing a base, spending money for the base and for each token sent [p.14-15 — Build a Base (Military)].
Army tokens are not listed among the 6 tradeable resource types (Energy, Raw Materials, Food, Consumer Goods, Services, Diplomacy) covered by the Trade action's Export/Import mechanics [p.13 — Trade (Economic)]. There is no rule allowing you to sell, export, or otherwise convert Army tokens on the board into money or other resources.
Watch out: If you ever run out of Army tokens in your color while producing them, you can use any available item as a substitute since they're meant to be unlimited — but this is about producing new tokens, not about spending existing tokens already placed on the board [p.15 — Produce (Domestic)].