Yes, a seller can accept or reject any offer and until the bank owns the home, it's the current home ownerâ€™s choice. I will go even further; a seller should reject an arbitrarily low offer on a short sale. Once an offer is submitted, in most places the status on your MLS will probably change and showings will come to a halt. Effectively taking a house off the market at a price the lender will likely reject is not a recommended strategy.
My suggestion is that if you are interested in buying a short sale, you should get a good deal to be willing to wait for an undetermined amount of time. However, to increase your odds of success, your offer should be at or very near at a price the bank will say yes to. How much below fair market value will depend on your area, the lender and how badly you want the house. Your local agent will be key to advising you on this.