I only use Points.com as a wallet that shows me how many miles/points I have in each account I have. The only times i've ever bought points or miles have been

1. Jetblue Points when you could redeem 100 or 200 true blue points for a RT.
2. US Airways Dividend Miles

Both times I did because there was a 100% bonus involved and it was directly from the airline.

I thought points.com was for trading miles with other members. I've never done it because its so expensive to do as others have pointed out, makes it kind of pointless to waste your time and money on points.com.