Answer:
When a seller advertises a property he/she is inviting potential buyers to negotiate a transaction. The seller cannot be forced to sell to some specific buyer even if they send a letter or email, or call saying that they want to buy the property.
Imagine that a seller sends out hundreds of flyers or puts an ad on a website, he might get dozens of calls or emails from potential buyers, but no buyer has any right over the property until they negotiate the transaction with the seller.