Keep them! Or the 15 bar, at least.
I have a 15 bar Oscar which spends most of its time in the box while I play several other instruments, but it's always there when I need it for something special and it hardly ever goes out of tune. You don't have to be an expert to make it sound good. It helps if you have a group/session where they play in one of your keys, so you can just quietly strum along until you find you've mastered your harp. If you play guitar, then you already understand chords and keys which makes it much easier. Then you can venture out a bit and go solo with confidence.
Don't sell.
Robyn