It mentions that the strap buttons were changed out, so the assumption that the electronics are original would be a reasonable assumption. The buyer probably has cold feet or wants to get some money back, but they will win any disputes.
Yeah this ^^ makes the most sense. The fact that the seller
specifically mentions the strap locks mod, which is a relatively trivial thing and had offered the OEM strap pins instead, but
fails to mention any other alterations, especially major items like electronics mods, is the same as saying de facto that no other alterations exist. So it's not just a "safe assumption" that the rest of the bass is still 100% stock...it's an unspoken affirmation.
The OP's trustworthiness is not in question here, he no doubt overlooked the Nordy pickup. But that oversight left him vulnerable to a buyer demanding a return/refund, even if it's for buyer's remorse or any reason at all. And that buyer will likely win any dispute by reason of seller's failure at full disclosure. Them's the rulz.
Looking at the Reverb listing, more pics might have helped, especially a close-up of the pickup so the little "N" could be clearly seen. And yes, as pedantic as it surely is, more detailed info always helps. Even explaining details like the truss rod working properly, neck straight, bridge saddles, screws, no scratchy pots, etc. Virtually every imaginable detail...mention it. It helps reassure potential buyers, and saves them the trouble of having to ask all those questions. Moreover, it reduces risks that can lead to what the OP is going through right now. Hate to be "that guy" but sorry, this chaos could have been avoided.
Good luck, hope things work out amicably, despite the buyer being an obvious dikk.