Disclose untested functions and intermittent problems
Some functions may be difficult to test: specialised connections, MIDI, effects loops, digital ports, specific outputs or behaviours that occur only under certain conditions.
If you have not verified a function, say so.
If a problem occurs only occasionally, describe it as intermittent and explain when it appears, if known.
Do not turn the absence of testing into a positive claim.
This precision protects both the buyer and the credibility of the listing.