What should I do with the memory cards and items I found in the used laptop bag I purchased?

If you know the seller, return it to him so he can give it back to its owner, or you can identify the owner through him. If you do not know the owner, and the disks and items are of minor value that people usually overlook, then there is no harm in benefiting from them without announcing them. However, it is better to give them away as charity. But if they are valuable and their owner would seek them, then they are lost property (Luqata) that must be announced for a year, after which you may own them if the owner is not found.