The easy answer to this question is as much as possible, but that is not exactly the whole truth either. Unfortunately, there are going to be many things about your business which you may find useful, interesting, or important and that potential buyers could simply care less about. It is important to have everything organized and accessible just in case it is specifically requested, but as far as dumping boxes upon boxes of paperwork on your brokers lap goes, it is completely unnecessary.
Tax returns and other financial statements
Again, important things to have available, but the figures can simply be disclosed in order to properly market the listing. There are many prospects that make a career out of searching for and rifling through businesses for sale. I can not say what their true motivations is but they seem to endlessly request all of the financial statements for any new opportunity that comes on the market so that they can spend all of their time reading through and picking apart anything not to their personal taste. I guess that everyone needs a hobby, but we value our clients privacy and this seems like an unnecessary invasion of it. The buyer should be able to take the sellers word regarding the financials, and then if they make an offer they are entitled to the right to add the contingency regarding confirmation of all said statements and statements.
Pertinent sales information
Some potential buyers are going to be obsessed about the sales data you can provide them. They are going to want to learn about customer acquisition costs, closing ratios, attrition, life time customer value, and even average spend or traffic count. It is well worth your time to check into these things if you are not already familiar with them and to then run through some preliminary calculations to get the figures that you may need. If nothing else, it will at least help you get a better understanding of your own business.
Those unsightly things
We all have them whether we like it or not, things that we would rather not discuss or admit to anyone else. But, it is much better to get everything out in the open as soon as possible so that an effective strategy can be devised to minimize any potential damage done. There is nothing worse then finding out undisclosed information from a seller while the deal is in due diligence, it will kill the entire transaction almost every time. As a seller, make sure that you tell your business broker everything that they may need to know in order to do their job effectively, whether it is going to help your cause or not. Often times, a price adjustment is the best course of action, but again this is something that needs to be factored in as soon as possible so that not a single prospect gets wasted. The business is only worth what it is worth and no amount of wishful thinking can change that. If there is a key piece of information that is going to inevitably cause a price reduction anyhow, why not just start there so that you can generate more leads to work on from the beginning?