When can player thieves attack you when trading?

[Beethoven]

And what happens at every level (1 star, 2 star, 3 star, etc..)?

[TheKnight]

After Level 40, Player thieves can attack on Traders, even if s/he would do 1 star trade run.

The Higher your star is the higher your profit is, Each star increases the level and ammount of NPC thieves:

1 Star: spawns 1 or 2 NPC thieves of lower level than the trader
2 Star: spawns 3 or 4 thieves of the same level as the trader
3 Star: spawns 5 or 6 NPC thieves of the same level or higher as the trader
4 Star: spawns 7 or 8 NPC thieves of a level higher than the trader
5 Star: spawns 9 or 10 NPC thieves of a much higher level than the trader