Well standard kit decals are fairly worthless in general, unless someone is really desperate for those specific ones.
Can't speak for the truck/racing decals without knowing if they're standard kit decals or aftermarket decals.
For example if the plane decals came from aftermarket , say, SuperScale, Aeromaster, Eagle Strike, etc., these are worth at least something.
Check to see if you have a local scale modeling chapter and drop by a mtg to ask.