Around here most shops will pay about 1/3 of current resale value on a trade-in or if they purchase your gear outright. (If anyone offers more than that you can be sure that they're inflating the price of something you're buying in order to make up the difference.)
For most recent (say, five years old or less) instruments and other MI gear, resale value tends to be between 50% and 75% of the new street price. That can be affected by things like whether there was recently a blow-out sale at one of the online retailers, whether the item is highly desirable yet no longer made or available only on a waiting list, whether the item has a poor reputation for quality, etc.
I don't think there's really any consideration for age unless the instrument is part of a manufacturer's product line that turns over frequently, replaced by more desirable items at the same or lower cost. Digital gear is a good example of this phenomenon. Pity the poor keyboard players...
Consider a guitar that was purchased new for $500 a few years ago and is still in excellent condition. Assuming that was a fair price at the time of purchase, and by that I mean priced somewhere around 60% to 75% of MSRP - this guitar would fetch about $115 on a generous trade-in around here. That may be disappointing, but it's not unreasonable. That's just the way resale and trade-ins work.
You could probably sell the same guitar to a private buyer for $300 with no problem. You might ask $350 and settle for $300 if the buyer wants to haggle.
If you have any music stores that'll sell your gear on consignment, you could ask (and accept) the same prices ($350 and $300), but the store will take between 20% and 30% of the selling price. The only time I sell on consignment is when I think that the foot traffic in the store will give me a shot at selling a slow-moving item or when I don't particularly care to have tire-kickers traipsing through my house.
I don't really care for EBay. You do get a much larger market and usually top prices, but the difference in selling price tends to get eaten up in fees. Then there's the hassle (and cost) of packaging and shipping and the slight possibility that a buyer will jerk you around. It's probably worth the hassle if you flip a lot of gear, but for occasional sales I'd much rather do a face-to-face deal - cash and carry.