my opinion on your queries are as under:
Is my given signed eligible for any legal action??
Opinion: No, this latter is not any legal entity, they can not do any thing wrong against you or not entitled to take any legal action against you
i signed letter due to their compulsion. Now can i start my own business in the similar field i was working for them?
Opinion: its your fundamental right to start your own business, no body can restrain you.
Can i buy product from the same supplier or sell to same customer to whom my ex-company do??
Opinion: yes, you can do that, no body can restrain you to do that.
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