I often use the in game interface but there are some caveats and some of my tips to this:
To get trade data on systems you need to have docked at them. This means that to be an effective trader you need to build a collection of data within a certain area/bubble.
The large pad filter on the galaxy map should work but I don't find it works accurately for Odyssey settlements so invalidates ground sites quite often. Just rule out surface sites unless you want to verify pad size another way or trade in a small ship (for fun).
Take trade missions rather than trading blindly. They're good rep builders and if you get them right, quite lucrative.
The market comparison tool in the commodities tab of a station is best for comparing profitability once you know your destination system.
