This is where extra information is very useful! Knowing that the seller sells medicinal plants can help in searching, although I suspect there is more than one Turnera species which is used medicinally there might be one species which is better than others.
Have you asked your seller where he got these seeds from? That would be a good clue if he could tell you that!
If you can get the origin of the seeds, then a search for something like "ethnobotanical Turnera country X" might get you further.
Look at the results I got from searching the terms using Surinam ..
https://www.google.co.uk/#q=et...
