Questions about starting a new page

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So I have never sold anything before, so I do not know anything about marketing, profiting etc. my boys have about 1,000,000 baseball cards they have bought over the years, I would like to help them start a page to sell these cards and gain profits for college, they are seniors in highschool. Is this a good idea? Just to sell one product? They would like to grab bags baseball packs of a 100 cards for 5.00 a pack. Ideas? Suggestions? Also I have been reading about putting affiliate links on your page to generate more profit. Is this something that is possible to set up on Shopify sales page? Just looking for some direction.