at the "Add Reply" page, at the bottom of the text box, tick the "Do not combine posts" option.

Tradezone. There are a number of subforums, but either the name would be self-explanatory or there would be a pinned notice that explains what that subforum is for.

You can sell your watches either at Street Mart, Stuff for Lads or Stuff for Ladies.

Do note that the maximum number of threads you can open per selling item is one in the whole of LYN, not one per forum. E.g.
Model A in Street Mart, Stuff for Lads and Stuff for Ladies = cannot
Model A in Street Mart, Model B in Stuff for Lads = OK

If you use the LYN uploader there are no limits, but it'll be best to use an external hosting site e.g. Photobucket as you can manage your photos better. If you use an external site, max photos per post is 10. If you want to post more photos in the same thread, click the Add Reply button (instead of using the Quick Reply box) then tick the box that says "Do not combine with previous post"

Either Stuff for Lads (since you're selling men's watches or Street Mart (since they are watches) will be fine. Please choose only one.