I'm working on a vintage 1980 Honda XR80 tank for my sons bike.
Here's some things to look out for if your tempted to bid on a vintage tank or find one used locally.
1. Do you have at least on clear picture of the inside of the tank? If light too no rust, move to number 2. If you see heavy rust on the inside, walk away and don't look back.
2. Does the tank in quesiton say it comes with a gas cap and petcock? Are both in 100% usable condition. If not having to buy then can jack up your real world tank cost.
3. Going back to pictures, how does the tank mount to the motorcycle it came off of? Can you "imagineer" those mounts to work on your bike in some way?
4. Visit a online vintage motorcycle parts seller and verify that you can get replacement gas caps and or the original rubber tank mounts. I discovered that some bikes, use a tank gas cap that you can't get anymore and because of how they mount to the tank, nothing exists in the aftermarket. Be carefull.