I have an excellent condition gas fuel tank from my parts car that I'm selling. It should fit any of the W123 gas cars. I'm asking $300 or best offer. It DOES include the fuel level sending unit so if you don't want that I can reduce the price. It doesn't have the rubber seals on the bottom but otherwise is in excellent shape.
I'm not sure what shipping would cost but the cardboard box I have it in is 18.5 x 15.5 x 57 inches and is 31 pounds. If you'd like to purchase it through eBay that would be fine as well.
Here's a link to the eBay listing:
https://www.ebay.com/itm/1981-Mercedes-Benz-280-FUEL-TANK-GAS/192879264405