I will try to help you to the best of my abilities to answer all your questions.
Whats a good amount of cash to start selling rsgp? 50 mil +
How much should 1m cost? Check current sellers and see their prices (USD), you most likely will need to build up vouchers and trust on sythe before many will use you.
Whats the quickest way to transfer? Depends on what level your account is, and if it is mems or f2p. If it is mems you can easily use the dragonstone method. If it is mems your accounts (with the money your selling) should be atleast in mid crator.
How much chance is there of getting banned on rs2? If you do it right then you should not have a problem, just make numerous mule (bank) acccounts. So if you do get banned you will notlose to much gp. And mems accounts normally only get a 10 day ban for macroing.
Whats the quickest way to build up money? Botting is a quick way, simply as you bot for 10 hours you can easily make 1.7mil (3 mems accounts X 1.7mil =5.1mil profit a day)
Whats the chance of me getting scammed? Play it smart and you will never get scammed, read the guides which are posted here.
Good Luck with your Gold Shop. I sure Hope i have helped you.