Generally speaking I either buy singles (often playsets of 4) of stuff that I'm actually going to use in a deck, unless it's specifically for an edh deck that only requires 1 copy. I tend to only trade with others when they have something I have been unable to purchase online, or if they really want something of mine and I know I either have spares or I'm not likely to use that card any time soon.

Everything else either tends to build up as part of my collection which I'm happy to keep and would prefer not to trade, as I do enjoy just having lots of rare/mythic cards, even if most don't get used, or at least, don't get used anymore. I'm particularly stingy when it comes to trading away things like lands, especially my fetches, as I know they're always going to have value to me, either price wise or for use in deckbuilding.

On saying that though, I have on occasion just bought something because I wanted it, with no intention of using it in a deck, most notably with my copy of
this guyhttp://i262.photobucket.com/albums/i...Progenitus.jpg, because why wouldn't I want this?