Questions about buying/selling rares.

I'm trying to get into buying and selling rares for profit, but one thing I noticed is, rare prices are sort of all over the place on the MP. Some stuff goes for average, but then there are some items that might only have 2 or 3 of available, and the price is above average. So would buying something for average, and posting it for more be the way to go? Or is this something that is frowned upon? As well, I seem to notice there's higher demands for some rares over other as well, like what's good stuff to invest in and stuff to avoid?