Quoted from too-many-pins:It happened to me yesterday. Machine listed locally for $200 OBO - my email to seller "if your machine is not already spoken for I will take it and I'll pay at least $250 for it (possibly more)". After she agreed to sell it to me I never got an email with her address. Today same machine is listed for $450. I emailed her back and told her I am no longer interested at any price.
I like your second response. I will definitely use that. I can't stand people who bait and switch or just keep raising prices or agree to sell and then don't because someone offers them more while you are on your way to pick up the machine.