1) I've never seen someone sell their Pneumag.
I'd guess fair price would be something like market rate for the gun + @$200.00 for the conversion. That's just a guess though. Your mileage may vary.
2) Weight, lack of modes, and lack of battery. The pneumag is all mechanical, and has a very light trigger. The weight of the components that the conversion adds is something like the weight of a nice thick ham sandwich.
An e-mag is significantly heavier, needs recharging, but has the capability of getting all the hyper-blastem-flatter modes that people run in tourneys today.
3) If you don't change your game play too much, you can keep your current loader. I actually don't run my Pneumag fast enough to need any force feed whatever. The 8-10 rounds a second drop from my plain old motorized hopper are just fine.
4) To this point my Pneumag has been pretty much maintenance free. Clean things up after a game and run oil through the system occasionally.