I'm talking about AlphaBay but I don't know if this applies elsewhere too.
Example: The popular vendor named "rs6" ships from the Netherlands.
Search for: 1G Cheap Cocaine
Leave all other fields to their default. You'll get one result, his listing. Now search again but change "Origin country" to Netherlands. You'll still get the same result.
Now try this again but with: 10G Dutch Speed Paste Amphetamine 80% AAA+
With all other fields blank, again, only his listing shows up. But now notice how when you set the country to Netherlands, you get 0 result.
It's on purpose. NL vendors are risking getting their clients fucked and money burned because they are too lazy to ship from somewhere else.