Yes, it can be frustrating to a buyer, but the process makes sense. Sellers have the burden of maintaining their business while at the same time selling their business--trying to keep it confidential and avoiding disruption of normal operations.
With the NDA, buyers will usually receive a detailed description of the business, details of the lease if applicable, recast P&Ls for the past two to three years showing the owner's true Seller's Discretionary Earnings or Seller's Discretionary Cashflow ("SDE" or "SDC"), a report showing monthly sales figures for the same time period, and an itemized list of furniture, fixtures, and equipment ("FFE"). The Buyer will usually have the opportunity to meet the seller and to visit the facility. This has taken a significant investment of time and effort by the broker and by the business owner for each prospective buyer--and there may be several at any one time.
The prospective buyer now has enough information to determine a reasonable offer for the business, everything else after that is for verification, not for valuation. This additional information -- tax returns, payroll records, customer lists, supplier records, inventory details, etc. -- is highly confidential and involves time and attention by the business owner. And, before the seller and broker invest more time with the buyer, the buyer needs to demonstrate seriousness about the transaction by making an offer along with a deposit check (usually 10%) payable to the escrow agent. This also demonstrates that the buyer actually has financial liquidity. After negotiating the offer, the buyer and seller reach agreement on price and terms--and there is a contract. At that time, the deposit check is deposited with the escrow agent.
But, the buyer should not be concerned; he just has to be sure that the purchase agreement includes a due diligence clause allowing him to cancel the contract for any reason or for no reason during the due diligence period (typically, ten days). Then, the prospective buyer gets to look at and ask the seller about everything; the buyer can have his CPA, attorney, and other advisors check the financials, tax returns, bank statements, equipment, personnel records, contracts, lease (and lease assignability), and anything else. Obviously, this is a big intrusion on the seller's business, and not one that makes sense to allow for every "tire-kicker"; that's the reason for the commitment shown by the offer and deposit. To be clear, once the contract is signed, the seller has an absolute obligation to sell; but, the buyer only has a contingent obligation to buy--and, the buyer controls the contingency. Before the due diligence period ends, the buyer can cancel the contract and receive a refund of every penny from the escrow agent.