Another option would be to not counter offer anything. This shows you are not desparate and that you recognize thier offer as a lowball offer. If they are serious, they will come back with a second offer. If they are not serious, you can call them a week or two later with your counter. With time still on your side, I'd not respond.
That said, it is a buyer's market. The market can be down 10% 3 months from now and you'd get $485 anyways. The call is yours, but the best way to let a buyer know they are not serious enough is by not responding, or by responding with a simple "no thank you" with no counter.
Not knowing the actual market you are in and more about your house compared to comps makes it almost impossible to tell you if it is the right thing. If your house is priced to market, it will sell. Houses are still selling. The entire country is not falling apart, regardless of what the media says. If you have had your house on the market for 2 weeks and already have an offer, you must have a desirable house in a desirable location.
Watch out for the agent pushing you to counter. Remember, his/her job is to get you to agree to sell. In most states, the real estate agent commission is due once there is a written contract and all contingencies have been met, even if the buyer never actually buys the house.