Hi E_fury!
Well, in a nutshell, player shops as you probably know are basically shops an alternative to selling your items. (excluding drugs of course) When you rent a shop, you pay a certain amount for more rent days paid in advance. Everything you build there stays there, including the items in the chests, meaning if your rent days run out, someone can claim it and take all of your items. This is something you need to be weary of.
On the topic of selling drugs in player shops, this is not allowed and will likely lead to a punishment from doing do. I believe this is because players who buy drugs from player shops are in an area where they can be easily frisked by cops if they do purchase said drugs. I also think that this could lead to another way of safekeeping drugs, selling them to he drug house/prison easier making it far harder for cops.
Hope this helps,
-theodor488