Yes, DL pins can be traded at WDW. I don't know of any reputable sellers on eBay, but they shoul dbe easy to find - check to see if they have a lot of positive feedback for selling pins. If you see more than one or two complaints, buyer beware!
As to how many to start with? That's a good question - it depends completely on you and your family. I plan to get at least a 100 pin grab bag for our trip this year - this gives 20 traders for each of us for a 10 day trip... which is a measly two pins per day. If I can justify it, I'll probably do 200 pins to take.
On our last trip, there were only four of us. We'd buy a 7 pin started set each day - the kids would each get two traders, and DW and I would take turns being the one who got 1 pin or 2 pins that day. However, we "cheated" several times and bought extra starter sets (use 1.5... even *2* per day).
But we spend a lot of time looking at pins. Others may not.