How to hide html ftp link location/ ftp security for multi users

I need help on how to hide html ftp link location and need help with ftp security for multiple users. i have a site were I bought a automated scrip to sell downloads from website or server options but I have thousands of files I am selling so I chose to just sell the html ftp link with user name and password to just the specific files directory which is displayed right after paypal payment has been maid. Everything is good on that end. But I would like to hide that ftp link and location and allow multiple users to be able to be logged in to the ftp user name at the same time.

I made the ftp link correct with the username and password included just so my buyers wouldn’t have to enter the password over and over every time there logged in or out. But I need a better way. This way works but would like to have expiring links and or some type of share proof system in place like say for instance s2member plugin for wordpress or something. But my way is set up noce and would like to just really be able to secure my ftp location. Because I like that people can just click the link and log into my ftp but if I cant secure it for selling I have to find another way.