I am rather in favour of this right of first refusal, which simply means that if somebody wants to sell, he has to send you his offer to sell or, if he receives an offer, he has to submit it to you so that you can exercise your right of first refusal. You have the right to either accept it or offer the same amount.
You mentioned that there have been only two properties that you have not managed to purchase. Did you still receive offers from these people?