I also like to make sure the primary and secondaries "match" the mod type (set type) and how I'll use it.
As an example, if it's a tenacity set cross with a potency primary, I'm probably not wasting my time/resources on it, because if I'm going for potency, I want it on a potency mod - the same for tenacity.
Similarly, a crit damage set with defense as a primary or defense mod with offense primary - most of those are being sold. There are a few outlier cases, but for the most part, it's better to go with secondary and primary stats the fit the mod type (and each other). You've only got a limited number of mods and stats to improve - try to make that as efficient as possible.