Her parents RAN a shop in the town and she WORKED there when she WAS a teenager.
We used Past Simple for a sequence of events that happened one after another
While she WAS SERVING in the shop in 1888, she MET Vincent van Gogh, who had come in to buy pencils.
We use Past simple for a single event that interrupt a longer event in the past. We use Past Continuous for the longer event
While she was serving in the shop in 1888, she met Vincent van Gogh, who HAD COME in to buy pencils.
We use the Past perfect for an event that happened before another event in the past