Ryobi battery getting old/weak?
Yeah, sounds like you are ready for a new battery. 3 or 4 years of use is not bad for any NiCad battery.
Unless you have some reason why you want a new drill, or you want to change to one that has battery compatibility with other cordless tools you own, then "if it ain't broke, don't fix it." Just keep buying new batteries every few years until you find some reason to upgrade your drill.
If you buy a battery from eBay make sure you test it as soon as you get it.
In my profession, we deal with uninterruptable power supplies (UPS) that have very large, expensive arrays of batteries. Buying a used UPS is a pretty normal thing but you have to be a LOT more careful about buying used batteries. People will rip you off.
I'm sure the same is true with consumer rechargeable batteries. Buyer beware.