You’ll need the exact same stuff you needed for the last year extension. You may want to visit your immigration office beforehand and get the latest handout regarding current requirements in case they have changed slightly from last year.
You can’t do any form of work within Thailand (paid or not) without a relevant visa and work permit.
That said, whether anyone would enforce that for a “yard sale” is debatable. If you have a Thai friend they could organise the sale on your behalf. As long as you aren’t “working” then there’s not really anything anyone could do about it.
With the change of visa exempt to 60 days and technically no limit on number of entries it sort of negates the need for an METV.
That said with an extensive history of entries it may be better to have one than not. A visa isn’t a guarantee of entry but certainly increases your chances of not being stopped and questioned.