Is listing a watch for trade only acceptable in the For Sale forum?
I'm not a moderator here, but I believe so, yes.
Once the listing is posted I am sure you can reference it when appropriate. I would also see if someone is selling or the watch you want to trade for and PM them with your trade offer.
Yes, there is a place to tag it as WTT (want to trade) or FS (for sale, obviously) or FSOT (for sale or trade) and you can put it in the listing that it's a trade.
Separate names with a comma.