What is the difference between the following sentences:
This product is sold very well.
This product sells very well.
If the meaning is the same, what pragmatic difference is there?
The meaning is not the same. (And I think you mean "practical" instead of "pragmatic" as well.)
That statement implies little about the product, and more about the person selling it. Nevertheless, it is poorly constructed and something a native speaker would not be likely to say. You could use the passive voice to say "This product is sold in pharmacies," or "This product is sold by the gallon," or even "This product is sold all across America." But you would not describe the sales volume in this way.
Instead, your second sentence works just fine.
This statement means that the product is very popular: a lot of people buy it.