Is my traction battery failing on 2004 Prius?

The car:
2004 Prius Gen II with 250K miles. New HV battery & inverter pump in 2018, uses some oil.
I'm interested in feedback on two questions:
1. Is my traction battery failing?
2. What's the best place to post notice to sell a 2004 Prius if I buy a new Prime?
#1. Battery history:
I'm the original owner and have replaced traction battery twice, first with a rebuilt one, then later with a new one at 229K in 2018. It has a 4 year warranty good until April 2022, $2,750 with Greentec Auto.
Also replaced inverter coolant pump in 2018 out of an abundance of caution/Greentec did not push this, $352.
In 2019, battery charge was fluctuating and I got Codes: P0A80, P3013. Greentec replaced malfunctioning cell(s) under warranty.
Car was fine until winter 2020, when I noticed the battery was cycling up and down more quickly and it seemed to be slower going up hills. Then January 2021, on the first trip after sitting one month:
error code: P0A80 Replace Hybrid Battery Pack. Lights showing: Red triangle, Check Engine Light, (!) VSC.
I cleared the code and it has not reoccurred BUT the number of bars in the battery display goes up and down rapidly, and I think the gas engine is racing sometimes to charge the battery (or maybe it is the battery fan)? This reminds me of the symptoms as before the failure in 2019.
I realize now I should not have cleared the codes and trouble lights, but I HATE to drive the car like that and I had to get home. I thought they would reoccur immediately and I could take that to Greentec, but it hasn't happened yet during other trips (10 miles to 60 miles). I think the battery is failing, but my brother says it was temporary due to car not being driven for a month. Any guesses which of us is right?

#2 Selling this car: with battery fluctuations, chronic P0420 codes, and 17 years of use, I'm shopping for a 2021 Prius Prime. It still has the original catalytic converter, and mileage averaged 43 mpg for mostly in-city driving in 2020, body is in good shape. It's a great car for someone but not everyone. The trade-in value sucks and I would rather sell to someone who knows what they are getting into. Where do savvy people like that shop for bargains?
Thanks!

 

The warranty is still good, and I will get it repaired under warranty unless I trade it in this week.

I thought I was buying a Toyota HV battery, just one that is installed by a different vendor. Can't remember exactly what they promised, but I'm not upset with Greentec as long as they replace any malfunctioning cells during the period of the warranty. The problem I'm forecasting for myself is that I say "my battery needs to be repaired" and they say "what's the error code it is showing now?"

 

And 10% of that price is Washington state sales tax.

 

Yes, the warranty can be transferred to a new owner once, as long as it is done within 30 days of the sale.