From the legal side it's a good idea to make sure you have the following available in addition to the other documents mentioned below:
-All corporate formation documents (articles of incorporation or articles of organization)
-Any partnership or shareholder agreements between you and other owners of the company
-Copies of all of your corporate records (minutes, resolutions, share ledger, etc.)
-If your share ledger is not clear, and many are not, a capitalization table showing who has owned what shares, with a separate column for each time there was a new shareholder/investment that changed things.
-Check that your entity (if a corporation or LLC) is in good standing, and if formed outside California that it is registered in California if need be. You don't necessarily need a formal certificate of good standing. Just make sure the entity is active so if the buyer does their due diligence you aren't in for a surprise.
The lists of standard documents below are a good starting point. If I am on the buying side I'll always want to see any employment agreements/offer letters/handbooks, any material agreements with customers or suppliers and explanations of any litigation if there is a history of it with your company. A buyer who is well represented will find out about any past or pending legal issues so it's worthwhile to be prepared and make sure you're disclosing properly.