For new skis, the best time is generally in the spring, when stores are clearing out overstock. They don't want extra skis lying around all summer, and once next fall/winter hits the new models will be in the shops and they'll have to sell the old ones at a steep discount. Of course, very popular models will probably be sold out at that point.
Occasionally you can find last year's models still in stores around this time.
Local ski sales/swaps can be a good source for used gear, since you can inspect it before buying. Most people sell skis long before they're actually worn out.
I've gotten some screaming deals purchasing (new) skis online -- 'Dawgcatching' here on Epicski is a prolific seller, among others.
Buying used skis on eBay is dependent on the seller, and knowing what you're looking for. Skis generally hold up well to abuse, and most issues can be fixed, so if the seller is reputable I wouldn't worry about it as much as some other types of gear (like, say, a climbing harness). You'd want to get them (and the bindings!) checked out at a shop before using them, obviously.