How to collect manga/merch in an affordable way?

Hey everybody,

I'm trying to start collecting manga and merch, but it's so expensive everywhere I look. I always see people who have a collection of so many different mangas with all the volumes. I usually limit myself to only buying the first volume of all my favorite mangas because it can get costly. How do people afford to buy all that manga and merch? How can I buy affordable copies of my favorite books? What about merch?

Thanks.

EDIT: Thanks for all of your comments. I will be on the hunt!