Sell side diligence, also called vendor due diligence, is the seller running the buyer's exercise on themselves before marketing. Its purpose is not presentation, it is discovery: finding what a buyer will find while there is still time to fix it or price it.
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What it covers
Usually financial first, because that is where the price is. Legal and tax where there are known grey areas. On a technology business a technical review. The output is a report you own, which means you decide what to share and when, and you know the answer to every question before it is asked.
When it is worth it
When several buyers will compete, because a vendor pack keeps them moving at the same pace and stops the process collapsing into one bidder's timetable. When the accounts need explaining. And when the owner cannot afford to be absorbed by responding for two months. On a small single buyer sale it rarely repays its cost.
What it changes
Buyers still do their own work, but they start from a document they must argue against. Findings arrive as a discussion rather than as a discovery, which is a much better position for a seller. And the fixable items get fixed at your leisure rather than under exclusivity, where every remedy has a deadline and an audience.
Questions people ask about sell-side due diligence
Will buyers trust a seller's report?
They will read it and verify it. Trust comes from the provider's reputation and from the report being candid about problems rather than silent on them.
Does it shorten the buyer's diligence?
Usually yes, materially, because the data is already organized and the obvious questions are already answered.
What does it cost?
It is scoped rather than listed. Ask two transaction teams for a written scope and price against the same brief.