Looking for advise on a massive undertaking

So... Pictured is box one of fifty/sixty and his hobby room but I'm working to help my grandfather sell off his entire collection over the next year or two to help him pay bills and cover what social security isn't. I'm currently cataloguing things A-Z, but my question is more what to use for listing things online or what the best way will be to get this all going. Ideally I need to link it to his bank account and all that jazz so he can start receiving funds as soon as cards are moving but it's more of less going to be a second job for me in the meantime. Am I better off setting up an e-commerce site where I can link things in and sell on eBay/other 3rd party sites or what routes have others gone in the quest for similar endeavor?

While getting things up I'll be buying some of these in bulk off of comps from eBay sold listings so he can at least realize some cash flow, but would gladly take any advise while I get this up and running. I work in IT Application Security and Development so it's easy enough to spin up a website if people feel that is the best route.

Thanks!