It's individual as to which point of demarcation you want to select, but I have found that in discussing runs with people, when I say that I have complete original runs of Avengers, Iron Man, and Defenders, people understand I mean 1-402, 1-332, 1-152. If anything, I have to clarify that with IM I just mean the self-titled series not TOS. I think the early Marvel reboots are a no man's land for many (though in fairness the Perez/Busiek effort was better than much of Avengers original run north of say 202. Past that only a couple isolated storylines stand out much.)
I include the annuals and giant-sizes in my definition; no one asks if they are there.
It's a process to define what's meaningful to oneself as a run to collect.
As to selling, I don't think it's an impairment to to break them up. I think for larger ones they might be hard to find a buyer for. I mean, who the heck likes the Avengers enough to buy that whole run from me without already having many of the books??
Defenders, I'm guessing, is about as large a self-contained set as I could sell in one shot. I suppose someone who has none might be inclined to buy and read it.
I have been on a kick of paring down the size of my collection and using proceeds/trades to acquire books I have always wanted that I never got. This has involved hunting down odd issues of many small runs to finish them off, then reading them end to end before releasing them. (Champions, Secret Society of Super Villains, Super Villain Team-up, West Coast Avengers, Freedom Fighters, etc) I will read the three larger runs end to end before making any decisions, but I have already traded a long box of Avengers-related books post-402 and haven't looked back. I reread a few beforehand is all.
I can easily see myself cutting Avengers off north of 202 (basically 200, except those next 2 are a great little Ultron arc; the comic book version of an Avengers novelette from a Marvel Superheroes paperback from the late 70's) after seeing what it's like to read the serial epic as a serial epic.
Then there's full sets that came within collections I acquired! These really stump me....I have the Spirit Warren and Kitchen Sink stuff and Jonah Hex from All-Star Western 10 thru Hex 1-18, plus a few later books. I'm reading, and I plan to release, but I still need a plan for those. Both are right at that size point (about a full magazine box) where someone might want them intact, but I'm not sure how to do it yet. I will probably walk them into the post office one day to figure out whether offering online is even an option (in terms of shipping).