I have a 1978 Mercedes 240D with about 238K miles. A belt for the coolant fan broke recently so it overheated, though it never reached the red on the heat gauge, and since then the coolant level has been going down, at first slowly, now faster, so I've been refilling it, though I don't see it leaking on to the ground at all, so I think maybe I have a blown head gasket, and that I'm sure is more than I can afford to have fixed, and I don't want to put anymore money in to this car, as I've replaced many parts already, and I'm sure I'll have to keep fixing more, it's old.
So if it ends up being that bad, (I'm taking it in tomorrow,)they are a Mercedes-only mechanic shop (not a dealer), so I thought it would be easiest for me to offer it to them as a parts car, as there are lots of new parts in it now, or a loner car which they offer to customers while their car is being fixed. They seem to be the most honest place around, so I hoped they might give me a fair offer for it, but...
Does anyone think I should sell it on craigslist, and would get more for it? Or because of the blown head gasket, plus other parts that need fixing soonish like the exhaust manifold (slow leak) and control arm/arm bushing, that I couldn't get any better for it from someone out there?

One asset it does have is a veggie oil tank in the trunk, plus all the tubing and best quality filter installed under the hood (just needs to be reconnected to the fuel line), which cost $1500, so that may make it worth more to someone interested in using that, and I know the mechanic shop is not interested in that, so it has no value to them (or so they would say, but then they could take it all out and sell it; and I don't want to ask them to take it out so I can sell it, because they said that would be $200-300.)
Thanks for any helpful input.
~Emily

As a rule, it's not a good thing to do.

Even if it needs work, you may be able to get more out of it from Craigslist than if you attempted to sell it to the shop.

Yes, seldom will the shop want it. The new parts once it is on your car is now a used part. And I doubt he has a lot of calls for 1978 car parts.

But yes some of the online sites, even a few of the national sites, if the car is good enough for perhaps restore

Thanks for your answers. It turns out it's not the head gasket, since there's no temperature fluctuation, so I don't have to sell it right away. Just needed a new radiator cap, it wasn't releasing the pressure it's supposed to, and I was over-filling it, it was draining out the overfill tube.

And Fr_Chuck, you'd be surprised how many 70's Mercedes you'd see driving around Portland, OR, so I'm sure they could use the car somehow, but I'll probably sell it online, when the time comes.

Thanks again, all.