Novice Trader by Ken Pence
Alan Einfeld is an apprentice trader on his first solo stint on Trappist 3 – in a slightly-behind-the-curve dimension. He is abandoned by his mentor’s death and now has little to trade and can’t get back to his dimension. He doesn’t know the language but the locals are humanoid. Finding an escape leads him to a grounded shuttle.
He doesn’t realize that a Pact among the fleet has restricted technology. He controls the war-time tech sought after by a group of rebels. His adolescent charges are developing too fast for their own good. How many Trappist locals would he have to kill to get back to the Prime dimension? He flees only to fight an interstellar war 100 years in the past.
An exciting romp through space and time with lots of confusion, conflict, conflagration, and cunning.