I am looking at an aircraft that has only 150 hours since a Lycoming factory major overhaul. Problem is that this was done 6 years ago. What kind of things should I look for to determine whether the engine sitting idle will be a problem? Should I avoid this aircraft entirely? Thanks for your advice.

Maybe nothing. If the engine was "preserved" properly for long term storage, then it may be in great condition. But if it has been sitting outside in the weather and not flown, then you could be faced with all sorts of headaches. I'd take and look at the logbooks to see what has been done. A good (independent) A&P should have a look at it. As we all know, aircraft engines like to be run. You'll get more hours from an engine if it is used more often.

Perhaps the engine is okay (no rust due to condensation), but you may find that it may not make it to the advertised TBO. Basically the engine might have a shortened life span.

ANY Lycoming engine will have the potential for RUST on the camshaft, due to its position in the engine case, it sits UP OUT OF THE OIL level on the top side of the engine case.
Also, could be rusty in the cylinders assemblies AND if the oil was not changed out during that period of time, you might have ACID build up problems and OIL breakdown. ALSO, the engine seals and gaskets may have dried up.
I would highly recommend the following : 1. BORESCOPE the engine 2. Pull off a complete cylinder assembly so you can get a look into the engine and view the camshaft surfaces.
3. If the engine is started and run up, get a AOA OIL SAMPLE TEST KIT and get a fresh sample of the oil after the engine is run up. DO NOT PULL the propeller through if the engine has not been started in all that time !!! GOOD LUCK !!!