I bought my kid's Guardian bike for $300 and sold it for $200.

For what it's worth, we had 2 Woom bikes and we found the resale value of them to be quite high!

Ive sold my kids older bikes on craigslist for $40 and they always get picked up. Try there. It just needs to be the right size, not much else matters.

I do really like the guardian when they are ready for gears though.

We checked REI and prices were ridiculous. I don’t know where you are, but look for a bike shop like Takoma Bike that sells used kids’ bikes. The nice thing is that you know the bike will be tuned up and ready to ride versus buying from somewhere like Facebook Marketplace. We bought used and I think paid around $200.