whats the big deal with strand maps?

With a good build no map should be a problem :) RF ftw :)

I've seen highlevels (90+) buying Museum maps and offering higher maps in return. Even traded some myself. I assumed there was some trick involved, maybe a recipe, but I couldn't find any additional info.

There's no trick, they just can safely do that map with a lot of mf. Three bosses = shitload of drops. Map lvl high enough to drop anything that could drop in a 69-71.
So they trade their shitty higher maps for that one:) They give them no exp and worth nothing anyway.

Best 71 map