Offers are not purchase agreements, but rather a method for negotiating the main contract terms (like price) before a formal purchase agreement is drafted.

If your offer is accepted, the seller will present a purchase agreement based on the agreed terms. A deposit will be required upon acceptance of the offer. 

Offers expire after 5 Business Days, The seller may counter-offer; which resets the 5 Business Days expiration clock. 

(Only submit an offer if you intend to follow through with the purchase agreement.)