SIM resale - Is SIM Card Good? | Buy / Sell / Trade | Consumer forum

When people are selling their SIM cards here, how do we know if the number is active and not ported to a different user? I understand the principle of buyer beware but we can't test it from across the country.

-How long does it take before it is de-activated?
-If airtime runs out before top up does that de-activate it?
I don't want to buy a "useless" SIM card so how do I check it?