Buyers and Sellers Have Different Relative Negotiation Advantages, and the Letter of Intent in an Engagement Helps Define Terms. Here’s How. While perhaps not the longest or most expensive document among those found in the in the M&A process, the letter of intent (LOI) may well be the most important, particularly to the seller. The LOI sets the tone for the transaction and serves as the road map for the due diligence and the definitive agreements. Ron Stacey explains.