Multiple offers headache

[Jenn Rosenberg]

So I recently made a (kind of low) offer on a house. This is where it gets tricky. The house has been on the market for 40 days, and the listing agent is claiming there are several offers and of course, that mine is the lowest. Seems dubious. My question is: Once the listing agent replies that he wants my "highest and best" offer, is he legally obligated to take it if it is the only offer? Or could the sellers decide to leave the house on the market even after they ask for "highest and best"?