Before I get asked I will beat you to it LOL.The reason why you only use YOUR pendulum it is really for your safety and to make sure it always works. You are the one who is telling it how YOU want it it work. YOU are saying I want YOU to do this for this answer and this for this answer. And you may not do it the same way I do. If you let someone else use it they may mess up your instructions and confuse the spirits and then you have to re-program your pendulum all over again. Then there is the safety issues. If you go through the proper protections before you go on any ghost hunt but some other person in your group doesn't then they may get hurt where as you would be protected right?
?? Same goes for your pendulum. You say the same protection prayer you would do before any ghost hunt before using one and I personally sage myself and my pendulum to keep away bad spirits away from it before I use it, therefore I am not going to just hand it over to everybody in the room. Plus after you have been working with one for a long period of time it gets in tune with your energy (or so others have told me) and yours may not work right for other people anyway. I always feel its better to be safe then sorry. Besides that you never know what other people could be doing with it! That to me is the only danger with them.
As far as the Danger, so far I have never had a problem with it, but I have never encountered a bad spirit with it either. But with my ring I was just playing around to see if a wedding ring would work and it did. I just asked it things I knew the answer to to see if it was right ...............and it was. I was not trying to contact the dead so it did not hurt me in anyway.
As far as sage, I get it at the powwows I dance in but you can get it on-line. I like the California White sage the best. You lite it then blow it out. Its the smoke you want to cleanse an area. Bad spirits don't like the smell. It is sorta a Sweet smell.