I'm not sure. There are only a few examples of other users doing this - but they too were not completely sure - and did not receive any blow-back from the posts as far as I can tell.

https://www.yamahasynth.com/forum/is-listing-an-item-for-sale-permissable,-here
which is related to ...
https://www.yamahasynth.com/forum/motif-xf7-for-sale
and a second one:
https://www.yamahasynth.com/forum/for-sale-yamaha-fw16e-firewire-card

Might be good to see an official stance on this. Adding an "items for sale" forum category would be a pretty clear indication and would keep the classifieds away from the more owner-usage-focused discussion currently found here.

Write to the Admin and ask them to open up a Market Place Sticky