It's not safe. The fact that it's already broken, beyond repair, increases the risk that there is some other issue that you haven't noticed yet.
So you need to factor in that risk into your pricing of the card.
How much of a discount makes the risk worthwhile? That's up to you. But if this card dies, and you sell it for parts, how much are you left with on the hook for?
The types of faults that could lead to a port not working:
- Mechanical, the pins are mangled
- Electrical, there was an over voltage on the output, a fuses blown, a capacitor's blown. Visual inspection may be useful here
- Water damage, some moisture got into the system which caused shorts and other circuitry damage.
- Heat damage, the card has gotten too hot and suffered some component failures. Maybe there's caps blown elsewhere
- Bad solder, bad joins, manufacturing defect, there's some electrical fault caused by improperly applied soldering
Regardless, if you buy the board, you should tear it completely apart, do a full inspection of it, take good photos, clean everything, apply new thermal paste. Put it back together. And enjoy