you can't buy a second hand genuine (i.e. not hacked) license without the developer checking authenticity and changeing API keys. so, the risk is absolutely zero. The owner can not modify it. Infact, the owner can not do diddly squat, except take your money ... which you may not give him, because it's not how things work hereabouts. There is an [EDU] about how to proceed in buying a second hand license in the link I already gave you. Please use it.