CMs don't pay for any pins they trade while working in the parks.
Some CMs of course buy pins for themselves, as there are Cast pins that only CMs can buy in a Cast store.
I think the "old" DS pins are nice. I own MANY myself. (That I bought for full price AT the DS when they were for sale. Mnay of us guests even stood in line for a LONG time, EARLY in the morning to buy.)
I suppose because at that time they were $4 + a pin. When at that time park pins were around $6+ a pin they could be considered cheap pins.
Either way, the Disneystore.com pins are tradeable on Disney property and are a very good deal!
There are some who dump literally bags of unwanted pins on CM lanyards through trading. (Bootleg, VERY cheap pins, and/or extrememly common pins no one wants, etc.)
But they should trade as long as an adult is not trying to trade from a child's lanyard, or they don't already have the same pin on their lanyard that a guest is trying to give them. CMs should not have duplicates of the same pin on one lanyard.
When you see the same pin sold in large (but cheap) lots on Ebay, personally I would never consider buying those to trade.