[Sheriff Gonna Getcha]

I need some more advice about real estate. Okay, so I am making offers on places and lowballing in the hopes that one bites (even if we get 10 rejections). But, I'm interested in actually outbidding another buyer for a home. The problem is that the Seller accepted that buyer's offer. So my question is, what prevents me from coming in and outbidding the other buyer? The offer sheets that I've signed doesn't seem to preclude the Seller from backing out, they ony seem to preclude me (as buyer) from walking away.