As a marketing manager
So my old boss don’t like all customers buy from me products
Why would your old boss not like that? You work for him and you’re good at making sales, which naturally means more profit goes into his pockets. I don’t understand.
Actully I am worked as a market manager
So my old boss fear that his all customers not buy products from me in future
Anyway…
You’re not going to be banned and immediately deported unless you were previously deported, overstayed your previous ARC when you left, did something illegal, are on some kind of watch-list, or walking through the airport with a giant sign that reads “ask me about all the cocaine I have.”
Your old boss isn’t some shadowy illuminati figure connected to the highest rungs of the Taiwanese government, is he?
If you signed a non-competition agreement, it may or may not be valid.
If you’re not sure, ask a lawyer or the Ministry of Labor.
If there’s no formal non-competition agreement, and you’re not going to reveal any trade secrets (see the Trade Secrets Act), your former employer should have no power over you.
In theory your former employer could have tried to have you blacklisted,
but it’s very unlikely.
Reminder to everyone: let’s keep the comments and questions polite and helpful. 
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So, you already in Taiwan? Then, why do you worry about immigration?
Is your concern about the blacklist @yyy said? If you are listed in the blacklist, I think it just means you cannot get a work permit. Nothing about entering taiwan.
Or, do you worry the case bellow?