Typically, as the close of escrow gets closer, the bond between the buyer and seller gets stronger. There becomes a point in the transaction when the due diligence is done, the checks with various government agencies is complete, and lease is being finalized, and so the buyer and seller begin to anticipate their new lives: one, selling the business and enjoying a short break, and the other being a business owner. With most restaurant sales, the standard amount of training period is 2 weeks or 80 hours.
Although, it seems like a good idea for the seller and buyer to begin training before the close of escrow, because (a) the seller wants to be out of that store as soon as possible, and (b) the buyer wants to learn as much as possible to successfully run their new endeavor - it's simply not a good idea. Many things can go wrong. It's possible the buyer is in the store on a slow business day, and gets nervous, and feels the average monthly gross sales have been misrepresented or feels overwhelmed running the store, and thinks it's simply not for them.
The longer the buyer and seller are together, the higher percentage of possibility a conflict of personalities could occur. It's possible that the seller introduces the new buyer to the employees, because at one point they will realize the store is being sold, especially if there is a liquor license and the posting has gone up in the window.
It s simply best to follow procedure and have training after escrow has closed, because the deal is never done until all the money is in escrow and final escrow papers have been signed.