Was in the same situation yesterday. Guy told me he was considering an offer on his machine that was a lower offer than i would have made (offer in my headbefore he told me this 7% higher). I wanted the machine but i wasn't about to pull the dick move and offer more to get it, just said if it falls through please get back in touch with me.
I lost the chance to buy it but i don't think my reputation would be very good if i took opportunities to pull the rug out from someone. Although i wouldn't want to mow it, it's a small world in pinball.
Machine's are not worth acting dishonorably over.

dup