Buy from a retailer but expect to have to learn to open the gearbox and give your AEG some love soon after depending on three things: quality of gun, luck of the draw, and your personal pattern of wear and tear.
Not even expensive high end guns are perfect and a few people I play with have told me that eventually every AEG has got to be opened up for care and maintenance, if not outright repair -- no matter how new or good it is.
Between my wife and myself we've got 4 AEGs and most of them have had to get some kind of maintenance. I'm finding that the more I jump in and learn how the gearbox works, how to maintain it, the less I worry about whether I've made the perfect choice.
As long as you get something that is relatively compatible with Marui parts, then a major failure from even a worn out used gun might not be the end of the world. You might just have to wait a day or two to get a spare part and you're off to the races again.
That said, with your first AEG you should go with retailers so that you can have some fun before having to maintain or open anything. There are some good retailer deals on the AV forums.
Hope that helps