So far and mostly yes! but there has been times when some stores and some kind of gc's don't activate or get de-activated for some reason
like amex gc's some times get blocked other then that all is ok.
But to answer your question more directly yes if you check before using
and it's active they are safe to use.
Please note, if you want to make a deal with this user, that it is blocked.
For the most part yes. Sometimes the gift card can be cancelled. But its just a gift card so not a big deal.
And to add about checking before going in. Its good to check. But some stores will still say there is a balance with phone check. And when you go to use it, it will be blocked.