The buyer probably doesn’t know much about Sonos.

They may have seen people talk about the different buttons used to differentiate Sub generations and think the same logic can apply to the One SL, but there is only one generation of the One SL.

I assume they want to see the serial number of the speaker.  I don’t know how to read the numbers exactly, but I think you can tell what year it was produced.  You could definitely tell if the speaker is a fake or not.

On speakers that are available for upgrade credit, you could possibly tell whether the unit has been used to get credit or not, but that’s not available for the Sonos One SL.

That makes more sense. I went for button rather than bottom…

The first four digits of serial number are YY/MM of manufacture.

Yea, I didn’t pick it up at first either, although I do want to start using ‘buttom’ as a word now.

can it be misused? 
since there is also a pin code printed…

That pin would only be “active” if they were adding it to their network, and had the speaker on that same network. You could easily obscure that data if you felt paranoid enough...or even ask them why. 

If it wasn’t a One SL, I’d be more concerned about the consumption of the discount than anything else.