GMTmasterHistory or GMTmaster1675 are great resources to get started.
You’ll need to be aware of relumed dials, hands and recut cases which have direct impact on price.
Always buy the seller, I would stay away from sellers you are not familiar with. The best watches I have bought are from fellow collectors I knew and more importantly trusted. Chrono 24 is a mine field, I’d stay away and proceed with extreme caution.
Best.