Lithium Ion batteries, if kept fully charged, lose about 10% of their capacity per year (numbers might have changed, but the principle is the same). If it's kept at 40% charged it'll lose a lot less of its capacity. Keeping them at elevated temperatures also degrades the internal chemistry, so if you've got a laptop that is kept on its charger a lot, and the battery pack gets warmed by the rest of the machine while the charger is on, it'll degrade the capacity. They also are rated for a number of full cycles, which can be made up from a set of smaller cycles (so discharging to 80% full and topping up to 100% five times is a full cycle of 100% charge). Given the degradation at 100%, deep cycling is probably better for the battery life than keeping it topped up.
Added to that is how good the batteries were in the first place - good brand names put a lot of effort in, cheap places, and what you may find on eBay, are going to be more of a lottery.