After you held presentation in the best possible way, more you can make nothing. Do not forget that different people have different tastes and different interests. Learn all lessons from experience of presentations (for example how to grind the presentation that it passed and seek to use most effectively them at communication with the following buyers.
Sometimes at sale of goods it is useful to present to the buyer of a guarantee of that he will be able to get the refund if it does not accept goods. Such guarantees are additional incentive in order that the buyer at once acquired goods. Practice testifies that the few people request return of money to them.