I bought my M4 from BuyAirsoft.ca, and I had a great experience with it. It was quick and painless, and I got a good product from them. I also found their stock and prices very nice for someone just starting in, and doing it on a student budget.
If you are going to be buying used, be very careful about it. My buddy did that and he now has two "display" only guns.
"Sero Sed Serio"