ESP32 FCC Certification
Here's a link in more general terms in which this question has already been answered

Kit vs Device:

No one on this forum can really give you legal advice, but from what I've read and my current understanding, if your device is sold as a kit, it does not have to be FCC certified. The FCC also has listed differences between intentional emitters and non-intentional emitters (read that link above for more detail).

Location: 

Of course, this can depend on where it is you would like to sell. If you are selling through a large chain like Walmart or Target, which it doesn't sound like you will be, then you would be required to give them information such as UL numbers, FCC certification, etc. 

Pre-Certified devices:

The good news is, most ESP32 chips sold from Digikey and other vendors already have an FCC certification, whether or not it's in a dev kit. That would at least help reduce the chance that you will interfere with nearby electronics.

In the end though, you want to provide a quality experience for the end user of your device, which means you would want to make sure you will not interfere with other electronics regardless of its legality. Sparkfun has a great videos on PCB design such as this one.