Hi,
I bought a Medela "Swing" Pump. It is a "single"--not "double" pump. It was $150 at Target. It is very small and lightweight. The pump, along with a storage bottle, fits easily into my purse. I've found it very discreet to carry around.
It can either be plugged into an outlet OR it can also run on batteries. It does drain batteries pretty quickly, so I bought a set of rechargeable batteries.
It has multiple settings and has "two-phase" pumping. It is a hell of a pump for the money.
I would not, under any circumstance, buy a used pump. Even the La Leche League discourages this. Milk can and does get inside the motor of the pump--it's happened to me several times--making it unsanitary and risky to be sharing a pump. Also, pumps do not last forever, so if you buy a used one, you have no idea how much life it has left. (New pumps from Medela have a 1 year warranty.)