MO Queston for mods regarding ads

[jdl71]

Please feel free to move this if there's a better spot.

What is the proper etiquette or protocol in this situation?

Member places for sale for a specific item.
Item sells, then other members ask if item is still available.
I have similar item that I have not listed for sale at the current time.
Can I reply to the original ad that I have like item?
Can I PM the member looking for item, tell them I have it and that I will place a for sale ad for it?
Or should I just place my own ad, and hope that the member looking for the item finds it?

I've seen this happen several times and have kept quiet as I do not wish to violate the forum rules, or feel like I'm hijaking the OP's ad.

A similar situation occurs when someone asks if the OP has a different item not listed.

I did not see this addressed in the sticky's on the classified forums, so if I've missed it somewhere I apologize in advance.