Some people might try to delude you with that, because they do not really sell do follow backlinks. However, it is very easy to determine if the backlinks is do or not follow, you should ask them for link to the subreddit where the backlink will be placed on, and then if you use the Chrome or Firefox browsers, you can use the right click on the place where your backlink will be placed and choose Inspect and then inspect the link code to see if there is a “rel=”nofollow”, otherwise it is a do follow link.

All do follow links pass on link juice, but whatever it will help you or not, it depends on which site you are trying to get listened, in this case it is Reddit and it certainly will not do any harm, since it is an authority site.