links are allowed as long as the link is directed to the item at hand.. ie you are in the vintage Motu Marketplace so if you want to advertise your ebay link to a vintage Motu item it's allowed.
If you link to your store or link to other goods that do not pertain to the marketplace at hand then it will be deleted.
hope this helps.
All guidelines can be viewed in the forums as sticky notes, if you have a minute have a quick read.. lots of great information regarding the marketplace and the org as a whole.
Have fun and good luck.