The age or place you purchased your items does not matter. As long as an item is eligible for protection, regardless of its age or if you purchased it used from a store or individual, you can add it to your plan and have it protected through AKKO. See what items are eligible for protection here.
For items you purchased used or through a 3rd party marketplace platform such as Craigslist, eBay, Facebook Marketplace, or OfferUp, request an email be sent to you by a seller stating the date you purchased an item, the amount paid, and what the item was.
This can suffice as a receipt should you be asked to provide one during a claim. However, for any claim of theft, you must also be able to provide proof of purchase (such as a record of a PayPal or Venmo payment, or a copy of your credit card or bank statement showing the date and amount paid). Read more here about how purchase receipts are not required for coverage.