Slightly different question about unique level at pickup...

So everyone knows that you want to upgrade to the next tier ASAP as it effects the level of uniques you find. And I've heard different versions of how (like armor level only affects armor, weapons and off hands are averaged (or it takes the lowest of either, or main hand affects main hand, etc).
However, I wanted to verify if PURCHASED uniques are set at a specific level? I was still running into common uniques in shops after I had upgraded to fine, so that was my working theory.
In addition, what about uniques that just... exist in the world? Not in a chest or dropped, but are placed, like stuck in something, or on display. The one or two I've seen were about my item level, however I was at the regular item level for the area so it might have been a coincidence.
Also, if you don't TOUCH the placed uniques, can you come back later and pick them up at a higher level?
What about uniques in chests? Does opening the chest lock that unique into a level, or can you choose not to take it out of the chest and then come back later?
I'm mostly interested because picking up a common unique and then breaking it down gets you much less (or rather lower quality, but since you can combine them, it is the equivalent) adra. So if I come across a unique that I don't really need right now, and would wind up breaking down for adra anyway, it might behoove me to wait and pick it up after I've leveled up my equipment, thus making it easier to upgrade more item to max tier.