Will a HV battery with replaced cells balance on its own?

There is no truth to that, and you should return the lemon for a full refund, get a partial refund and live with the problem and repair expense, or just plan on fixing/balancing the battery on the 3k vehicle you've purchased. If you trust them (unsure given the above), you might let them keep the vehicle for a day or two, so that they can fix/repair and balance the HV battery.

Did you get a carfax, or check the mileage and history by running the VIN at http://toyota.com/owners ?

 

If you put the car in drive with foot on brake and emergency brake on and floor the gas pedal you can force charge the battery and perhaps if you did that 4 or 5 times with taking an hour or two break or even a day long break in between you might be able to get it balanced enough to keep the Red Triangle away for a little while.

But the problem is that when you replace battery modules they need to closely match the ones in the existing pack. There's several things you can do to achieve this but they're more time consuming and costly then what I described above. If you like to work on cars I can walk you through it all, but if you need a mechanic to help in your region we could probably find one on here for you.

 

I read that the hybrid battery modules can balance themselves, but that was only on an ebay ad that was selling battery modules…

My guess is that they do not balance themselves or else people would not be buying battery balancer/reconditioners for that purpose.

 

By the way, does anybody know where I can find a balancer/re-conditioner to borrow in the LA area? I live in Ventura, California.