Keep the purchase agreement, closing or completion statement, and the schedule showing deductions taken at closing. Add corporate resolutions authorising any distribution, tax returns or assessments covering the gain, and bank statements tracing receipt through to the investment. Where documents are not in English, arrange certified translations rather than summaries.
Index the file by transaction step
Order the records chronologically — acquisition of the business, operating and tax history, the sale, distributions, transfers — so each document answers a question about money moving. Mark which items are final and executed and which are drafts, because a signed agreement and a term sheet carry different weight. Where a bank cannot supply older statements, ask now and record the request.
Missing items should be listed openly with a plan to obtain or replace them. Add one document to that index that is easy to obtain while the transaction is fresh and awkward afterwards: a short written confirmation from the professionals who acted on the sale — the accountant, the lawyer, or both — setting out what was sold, by whom, for what consideration, and how the proceeds were distributed. It costs little while the file is open on their desk and considerably more once the matter is archived and the fee earner has moved on.
Keep it with the executed documents rather than in correspondence. Hypothetical example: a seller obtains a one-page confirmation from the transaction solicitor within a month of completion, and two years later that page answers, in a single paragraph, a question that would otherwise have required three institutions to search their archives.