The postcard itself might not be that rare or valuable, but it could be worth a lot because of what's on it. If it has a super valuable stamp on the back, that could make an old postcard worth money.

This 1 V. postage revenue stamp from West Refaim was postmarked in Virikoso in South Giantsland 100 years ago. Problem is, none of these places ever existed. There is a second postmark for Nineveh ...