I find that the number of commons I actually use is much less than the number of commons I actually have, so I've left them all on my other account since it was a cluttered mess. Now I only get a common when I'm actually making an avatar and sure that I want it for that avi - the same policy I have for cs items. I think it helps in saving in the long run.
If I like all of the set pieces, I do tend to buy them... But it's rare I'll actually equip more than one piece at a time. There are enough items on Gaia now that even with more unusual colors it's not hard to find a variety of pieces that match any particular thing. You just shouldn't have to resort to sets these days.
It's very rare that I'll buy a whole set. The most I have of an item in my inventory is 4 and that item is Daughter of Snows because I really love that item! heart But, other than that, I have two of a certain item that I like. I do like to mix up an item with other items, so I try not to kill the set thing.
I did have four Kottan Bells, but I shipped two of them over to my mule, so the avi over there could have some cool clothing.
I'll usually only buy one part of a set, but I'll buy it in all the colours. I'd spend way too much gold if I tried to get whole sets, especially since I use the staples more often anyways (cinches, for examples).
I avoid using sets as much as possible. Occasionally I've had to change my avatar because I realized that I inadvertently used two items from the same set. Always an unpleasant discovery, but usually not too hard to fix. The closest I get to set usage is using an item and its recolor.
Depends on the set because Gaia tends to vary in tints of colors when making items so alot of times pink doesn't match another purple or pink doesn't match another pink. Also some sets are pretty nice on their own too. I mostly prefer mix and match or use a set and add to to make it look unique.
Honestly, it depends on the items as to whether or not I purchase more than one item that is supposed to go with another. Oh sure if I like a piece from a set that has several variations in color I'll buy more than one of those if I can use them in several looks. About the only time I'll buy pieces of a "set" is if it's a multi posed item that contains an awesome wig or small accessory like glasses or earrings and also has a piece of clothing I might want to use in an avatar. It's why I own two Zodiacls and two Urns. There's great small pieces in both items that I find myself using two of them at the same time. Sometimes you just can't help but have more than one when it comes to multi posed items.