ISBNs are issued to publishers. In order to obtain ISBNs from a regional ISBN authority, individuals and organisations have to be registered with them as a publisher. There is no great procedure to go through to be registered with an ISBN authority as a publisher. But being registered as a publisher with an ISBN authority gives you an identity that becomes part of any ISBN numbers that are issued to you.
Part of all ISBNs refer to the identity of the publisher that they are issued to. This means that if you purchase an ISBN from someone other than an accredited ISBN authority with whom you are registered as a publisher, then they will have some other publisher's identity encoded in them. ISBNs are therefore not transferable. You will not be the publisher of any work that bears such an ISBN.
So it is not safe to buy an ISBN from someone else. They are not transferable and will always be 'owned' by someone else. It is they that will be the publisher of record. In order to retain all your rights, it is necessary to obtain ISBNs from your regional ISBN authority, also see the Where To Get Cheap ISBN Numbers page.