This guide will show you a really easy and safe way to buy a pin, while still using an MM. You don't have to give the MM your password at all. He can't do anything to your account except apply the pin.
I will be only doing it in two browsers right now which are IE and Firefox since they seem to be more popular than the others.
doing it on internet explorer
*note, for IE it doesn't matter what country you pick, you still have to do the same thing.
Step one: The buyer of the Pin goes to
www.Runescape.com and click subscribe or
http://www.runescape.com/members/subscribe.ws. Same thing.
Step two: Choose the country, PBP or SMS, and then log in.
Step three:Once you are logged in and you see the page where you put the number in for the pin right click anywhere on a page (not a link or anything just a blank space) and click properties.
Once you have click properties, locate where it says "Address (URL)" it should be the third one down. Make sure you copy the WHOLE URL.
Step four: Now give that link to the MM, and he/she will paste it into his/her browser and make you a member.
Doing it on FireFox
* Here is where it got a little tricky. When you chose the UK and Germany as your country it didn't just give you the link in the address bar. It does for the United States, New Zealand, Canada, and Australia though.
Step one: The buyer of the Pin goes to
www.Runescape.com and click subscribe or
http://www.runescape.com/members/subscribe.ws. Same thing.
Step Two: Choose the country, PBP or SMS, and then log in.
Step Three: Once you are logged in and you're at the page where you put in the phone number, right click anywhere (not a link or anything) and click "View Page Info."
Once you've clicked view page info click the tab that says "Links" and right click and copy on the one that says "Submit Phone Number and Continue"
Once you've copied that link just give it to the MM and he/she will apply it to your account. Not much else the MM can do.
Hopefully this guide helped and made it a good bit safer for you. I encourage you do use this method when dealing with a unofficial MM, even an official if you like.
If you find any errors or something I may have missed or a suggestion, just post here or PM me. Thanks.