Underpriced my car, what's reasonable etiquette?

Interesting.
I'm no expert in online automobile private party selling etiquette but I feel that if you haven't responded to any of the inquiries you're under no obligation to sell at the advertised price. People are allowed to make pricing mistakes, learn and go on.
I think you can't communicate with potential buyers and admit you believe you under priced your vehicle and ask them to make a better offer. BUT..
I think you can ignore incoming offers and existing offers on that listing and RE-LIST at a higher price.
The challenging thing of course then being, potential buyers are going to probably recognize your vehicle from it's previous posting. So it may slow down the selling process.
2nd at bat you might get a lot of "Isn't this a vehicle you offered for less?" inquires. But so what? You're the seller. You can be honest and say, you just felt you made a mistake pricing too low, and now this is a more representative value.
You might lose interest, it may take you longer to find a buyer, but I think that would be the way I would handle it.

 

what is the kbb.com value?

 

I can't help you, but gosh, I think I might have to sell my car in WA. $5.5k would be above the top end of what that car would sell for here!

 

You need the money. Delete the post, and repost with a higher price. You don't owe anything to these other people.

In fact, many of them will likely "flip" the car for more. Lots of car sharks out there looking to buy low.

I can't find a nice Gen3 Prius in SoCal for anything under $8k... most are ex-uber cars with 175k miles and in rough shape.