If it's not running, the buyer has no way of knowing why, even if you claim it just needs 'basic maintenance'. So the buyer would have to price in a large amount to get it running properly. I'd say ~$15K less than the equivalent running car. It's hard to say much more based on your minimal description.
Maybe one systematic approach is start with the value of a #4 (daily driver with blemishes) on Hagerty and then subtract. So if it's a 1969 280SL:
https://www.hagerty.com/apps/valuationtools/1969-mercedes~benz-280slHagerty says $37K, minus $15K non-running, etc on downward depending on how much worse than a #4 you have. Here's Hagerty's description of #4:
#4 vehicles are daily drivers, with flaws visible to the naked eye. The chrome might have pitting or scratches, the windshield might be chipped. Paintwork is imperfect, and perhaps the body has a minor dent. Split seams or a cracked dash, where applicable, might be present. No major parts are missing, but the wheels could differ from the originals, or other non- stock additions might be present. A #4 vehicle can also be a deteriorated restoration. "Fair" is the one word that describes a #4 vehicle.