2002 134k miles for sale Las Vegas (needs repair)

Hey Prius Folk,

I've got a 2002 Prius for sale. It's been well taken care of, oil changes on time and I changed the transaxle fluid shortly after I got it at 107k miles (also replaced fuel pump / tank and dealer replaced invertor coolant pump). The tires are in great shape (2 are brand new). Everything worked and I got great mpg until two weeks ago.

Now I have a red triangle exclamation point light and it seems to be in "limp mode" (I can still drive, but it's sluggish once the light comes on). I charged the little battery up (because it's the original and it needs replacement) and it's perfectly driveable for half an hour or so before the triangle comes back. Could this mean it just needs a regular car battery (because keeping this one alive is straining the traction battery)? I don't know. Maybe it's the traction battery itself at fault, but in the 30 minutes between codes it charges right up on the display in 10 minutes or so. I do know there are some hardcore people here who can fix anything, and I would rather not junk a car when people are going to need parts for these into the foreseeable future, or when someone might put the time into it and have a great time putting another hundred thousand miles on it. Everything works beautifully until the light comes on. I'll have to run the error codes again to post them accurately.

Are you an environmentally conscious person? Great! I committed a terrible sin by switching to a Grand Marquis. I'm now responsible for several hundredths of an inch of ice caps we'll likely never get back, and if you fix my Prius and drive it, you'll offset my mistake!

Seriously, I'm happy to answer any questions, I'll be 100% honest with anyone, I just don't know how to diagnose the issue without Toyota's software.

Make me an offer, I'll be happy to let you come see / drive the car! Wait, what's that? You need parts? Well, if nobody buys it as a unit I'll take off whatever you need and sell it to you, so you can save your car from the same untimely demise.

Thanks!