You can sell kits, because companies do. Kits are only required to be authorized in very specific rules for specific types of kits. However, devices assembled from kits are not exempt from FCC authorization requirements. Home built devices not assembled from kits do get an exemption. You figure out the contradictions here - i.e. this is not advice.

The FCC has fined people for assembling and selling uncertified radio transmitters from other's kits. Like this guy: https://docs.fcc.gov/public/attachments/DA-12-574A1.pdf Note that the company that sells the kit he was assembling is still selling the same kit that results in an uncertified AM-band transmitter when assembled.