Essentially cards that are verified by our Research Team are placed onto the Ladder where you can view each card in a profile to get in depth detail about the card including a full sales history where each and ever sale has been reviewed by our team.
Sales History is simply where ALL sales in our database are placed for your to easily search. The sales that are populated in here may also have been verified on a Ladder card profile and you will know this by the green check on the result page.
Our Research Team verifies thousands of cards and all it's sales as explained here. These cards have in depth research and value estimation outlined in the profile of the card. The cards that live on the Ladder are used to build out our Indexes and are subsequently used to help determine the value of all cards in our software.
More plainly Sales History is where we house all sales in our database and Ladder cards are a subset of those sales that are verified by our team.