Don and the other agents made great comments and suggestions.
If you were shown the house in question, by the listing agent, and he/she is not available to help you write your offer, OR negotiate one he/she already prepared for you, then call the agents broker ASAP, and ask that they take over while the listing agent is not available.
ALL contracts must be presented to the sellers in a timely fashion (within 24 hours of writing). Now how long it takes for the seller to reply, yes/no or counter offer is up in the air.
IF you have not looked at the house in question WITH THE LISTING AGENT, I agree with the others. Get yourself your own Real Estate agent who will be representing YOU. All agents can show, and sell, any house that's listed in the MLS, regardless of if it is their listing or not.
The listing agent who sells you their listing now represents both the buyer and the seller (called dual agency in NJ). You should be informed of this agency relationship right from the beginning. It's no advantage AT ALL to buy through the listing agent.
Here's a sample of my company's dual agency consent agreement, explaining what we CANNOT do as a dual agent. Pay attention to the item number 4 and the last paragraph:
1) We cannot disclose confidential information that we may know about either party without that partyâ€™s written permission.
2) We cannot disclose any price that the Seller may take other than the listing price without the written permission of the Seller.
3) We cannot disclose any price that the Buyer may be willing to pay other than offer price without the written permission of the Buyer.
4) We cannot recommend or suggest an offer price to the Buyer or a counter offer price to the Seller.
Notice: The parties understand that any agreement between Seller and Buyer, regarding the final sale/rental price and terms, is the result of negotiations between Seller and Buyer acting in their own best interest and on their own