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I interpret the rules as you do. Selling part 15 equipment, or for that matter any electronic products as defined in part 15 rules without certification and labelling is not legal. It seems the rules are very broad on this subject, but it also appears that enforcement is targeted to devices which are intentional radiators. For example, I am looking at an inline power supply for a USB drive and it is not FCC labelled.
If you check the link below for “NAL’s” you will see examples where the FCC has cited people for this, especially for CB radios.
One interesting current case involves Ramsey Electronics. The equipment involved clearly was not legal for part15 use as transmitting devices and I noticed that the FCC mentioned that this equipment could not be certified under part 15 for reasons of frequencies and powers involved. They do sell a popular part 15 FM transmitter kit which is not certified and they were not cited for this because the rules allow sale of such kits.