This is something that it’s common all over the world, but I want to tell you specifically about Porto.
When I studied there, back in the days, there was always a group of guys outside the train station who would approach you to sell some drugs. (I know for a fact that this is still going today)
They would usually sell hash/weed and they were always insisting to go with you somewhere else so you could try it out. Now here comes the tricky part: they would always let you smoke something really good, and sell you something different.
One time a friend of mine ended up buying them some hash and when he got home it was pressed coffee inside a thin kind-of-shell of real hash.
If you want to buy some drugs, my advice would be to don’t ever buy them from guys who approach you. Instead try to be the one approaching someone/some group that you notice are smoking, and ask them if they sell it. In most cases, the worst thing to likely happen is that you will overpay the stuff.