No you shouldn't feel obligated, but at the end of the day there is a figure that you will walk away with after selling the home, whether you pay their costs or not. Your Realtor should go over this with you when reviewing and considering offers. Either that bottom line meets your requirements or it doesn't.
But beware of dismissing offers like these. They are pretty commonplace, especially for homes priced $250,000 or under. You turn down too many offers and you don't know when the next one will come along.
Utilize your Realtor and ask a lot of questions. The answers will come to you.
Best of luck!