Oh it has significantly more sensors than that, trust me.

As for the seatbelt sensor, honestly depends on the cost and how long you plan on keeping it. Without knowing your car, I can't say for sure, but I suspect it's a simple switch. The issue may very well also be in the wiring, or the switch could've broken loose or something (in which case you'd likely need the belt buckle receptacle thingy).

Personally I'm so annoyed by things like that, I'd fix it. It probably annoys you, and if you do end up selling it rather than keeping it, it'll annoy the buyer.