You my friend are a douche bag, for pulling that. I like most people understand wanting to maximize your return on investment, but as stated you both agreed to 750 before the 800 dollar offers, By doing what you did, u not only screwed a poor girl who took the time to meet u and not only that you had the audacity to tell her how to manage her money, sorry but you never tell anyone how to manage money that isn't yours unless they are paying you (consulting work) to tell them what they should do, you also showed how poor your character is along with how unbelievably unprofessional you are in business dealings, also you basically tarnished the rep of ppl who actually are good sellers and stay true to their word.
My advice to to you on how I would have handled the situation, you list on craigslist for 800, girl calls you and says 750, obviously you agreed to the offer, she says she's on her way, during her commute you receive offers of 800, at this point this is where YOU TAKE DOWN THE CRAIGLIST LISTING!!!! Also this is where you should have had the morale compass to say i'm sorry but i've already agreed to sell it to someone for 750, however if the person does not pick up the item in the next 24 hrs I will contact you. The whole she was late an hr arguement is irrelevant, you should have held onto that iPad for 24 hrs from the initial agreed upon price, 24 hrs would have been sufficient time for her to pick up the item/notify u that she would not be picking it up. BTW 24 hrs wouldnt have made a difference you would have been able to unload that iPad just as fast.
Really did u need that extra 50 bux, nope probably not as u initially agreed to 750, also did u really need to get rid of that iPad 2 that very day/hr, was someone dying or some pressing emergency where u needed to get the money so fast... my bet is nope.. u were just being a greedy capitalist. While there is no problem with this the problem lies within ur business ethics...It's a rule you dont reneg once u agree upon sale, unless there is a significant change that compels u to do so, and even then you notify the initial buyer of that, if it was 500 bux she would have understood, but 50 bux cmon.. there's no defending what you did and this is coming from a business owner myself.