I am going to sell my 3rd generation iPad at some point in february. It is a risk but if you can get by without your iPad for a while then I see no problem.
The iPad 4 is only a few months old and is being refreshed next week. The iPad 5 won't be out until after the summer and after the iPhone 5S.
Sell now, but you'll be sat waiting until September / October at the earliest. Besides the resale value holds up after the new ones are announced mostly anyway so why sell now regardless.