IME, if you find a good vendor, you get the substance that you asked for, ie if you buy LSD you will get LSD and not say, DOM or DOB or an NBOMe class compound.
As far as dosing, you may get less than advertised but it's not likely if you choose a good vendor.
Be aware that most if not all darkweb markets and some other websites that you might use to get the official links and look at vendor reviews like Dread are under constant attack by hackers that want to get paid to let them operate.
So sites could be up and down and make it difficult for you to browse and make a purchase.
I would wait for a few weeks while the issue gets resolved as some skilled people are working at developing a system that reduces the impact of these attacks.