I found some old 80 cent airmail stamps. Is there any reason I can't use these to mail books under 13 oz? They have a silver airplane on them next to the number 80.

Yes, you can use them.  I believe that airmail  doesn't exist anymore.

I've used my old ones without any trouble.

I think the stamps are quite a bit more valuable than 80 cents, and this goes for unused or used.

You can still use them for postage, but I would keep them as they are collectable, especially unused!  If you really want to get rid of them, I would glad buy them from you. 

Yes, you can use the old international stamps for domestic mail. Sometimes I buy older international stamps because of the picture on the stamp to use for my international friends just adding additional postage. So you should have no problem using the 80 cent stamps. 

I would love them for my stamp collection! It is so hard to get real stamps! Jean

I have several used stamps from different countries.  I cut them off the envelope.  I had been keeping them for someone but they no longer collect them.  So if anyone wants them, they can have them FREE with any order from my shelf. ... just PM me.