Is it legal to sell a device with radio in the US without FCC certification?

In the EU it's illegal to sell without external certification if the product is a medical device, is made for children or contains a radio.

Certification will typically cost you $5,000-$15,000 depending on which lab you use.

Modules like ESP32-WROOM, Seeed XIAO ESP32C3, and ES2810AA2 comes certified, re-certifications should not be needed(IANAL).

I believe that to be true (at least for the European market), however you do need to have the manufacturers documentation on it!

How does that works if you're just reusing off the shelf bluetooth component? (that's likely already certified)

Off the shelf radio module, ain't nobody got time for certs.

EE here - he technically has to comply with FCC Part B since he's selling in the US. That said, the hobbyist keyboard community has been pulling this for a while for small group orders without issues. Pre-certified modules are helpful, but stuff gets weird when it's on a board, so no guarantees. The fact that it's on a single layer board doesn't instill confidence since that typically means less than ideal return paths that can cause problems.