There's no reason to smuggle illegal goods. The profit margins aren't any better as if they were legal and there's always the risk of a fine for smuggling illegal goods. I don't know how it is on illegal goods as I've never been scanned with them, but stolen goods the fine is massive, even worse than murder. Last time I tried selling Tobacco in a system it was illegal in and I only made 300cr/ton profit. The animal meat that was legal to sell there gave me 500cr/ton profit. There's no bonus for illegal like there was in FE2, and that's basically required to make black market trading worth it.
Heck, selling a ton of stolen, illegal narcotics gave Omar Little $400,000, and here I got 33 credits.
Stolen goods are worth selling, as you didn't pay anything for them and they sell for half their station price. So 1 ton of stolen gold is 5000 credits, and 5000cr/t in trader lingua.
You'll quickly pick up what to pick up. Don't loot stuff worth just a few hundred cr, don't loot limpets unless you need them.
For example I just looted everything I found except biowaste and other really cheap stuff while hunting an ANARCHY <- really important! nav point in my 32t Cobra and it gave me 30,991cr profit. Almost 1k per ton. On top of the KWS bounties.