A rough idle and poor fuel economy point to a few things. Depending on your stomach for chasing things down on a car you're about to sell, here's what I would look for, starting with easiest/cheapest first:

- vacuum leaks. Very high probability, based on the bad idle you describe. I don't recall the intake path on a K-Jet system, but check the obvious places for vacuum leaks, including the intake manifold gasket. On K-Jet systems, the injector seals are a notorious cause of rough idles, but they're usually not bad enough to reduce gas mileage grossly. Spray some carb cleaner around each injector (where it goes into the manifold) one at a time with the car idling. If the idle changes, that's *A* problem to fix, maybe not the entire problem.

- by all means, pull the distributor cap and rotor and inspect them for arcing and wear.

- check the ignition timing

- any dragging brakes?

- if the engine labors to rev in idle, can't get over maybe 4000 rpm, you've probably got a timing belt that jumped a tooth.

- pull the spark plugs and look at the tips. Burned? Whitish? Soaked with oil? Covered with fluffy carbon?

Just a few places to start.
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Thank goodness we don't get all the government we pay for. -- Will Rogers

I'm sorry for your poor experience with your 240. 122,000 mi. seems mighty low for a 1983 model, most would say it was just broken in mechanically.

I don't think you are going to get much more money from a buyer if you fix the mechanical things you've listed, maybe $100 more running well?

I would just see if anyone wanted a fixer upper and sell it to them.

with that low of mileage you might be better off parting it out if you wanted to take the time to disassemble stuff.

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Bad Blue - '86 245 with 303,000 mi. (now for parts), Good Green 1990 245 with 178,000 mi., Blacky - '91 740 wagon with 180,000 mi., Mean Green - '94 850 with 222,000