If in fact you have just made an "offer" and you do not have a signed binding "contract" with the Seller, then typically the Seller can consider any other offers. Anytime a Seller receives an offer, they can accept, reject, counter, or ignore the offer, even it is full price.
You might be in the middle of negotiating with the Seller, with only a couple of issues to work through, and another offer comes in, the Seller can drop you like a hot potato without giving you a chance to meet or beat the other offer.
In many states, even if you verbally agree to all terms with the seller, the agreement isn't legally binding unless it is in writing and signed by all parties, which is why Realtors want to get the agreement in writing ASAP.
If you DO have a binding contract with the seller, the seller can still market the home and negotiate backup offers, just in case you don't close the deal. Backup contracts are rare, as most contracts do close.
I you need a referral to a good Realtor in your area, let me know.