If your most qualified prospect submits a Letter Of Intent that includes a lower price and more generous terms than you had wanted or a less qualified buyer has submitted a more attractive LOI, then now is the time to negotiate with the stronger prospect. It's almost impossible to improve the deal once you have accepted (by signing) the Letter Of Intent. However, the buyer may find reasons to re_negotiate a lower price during the due diligence phase.
2.) Some buyers will want to skip the LOI and instead submit a full purchase contract. They figure, why should they pay their lawyer to draw up both documents. But a purchase contract requires a significant amount of time and attorneys fees to draft. Once the buyer has made this big investment in legal fees, it puts a lot of pressure on them that is detrimental to the negotiating process.