This is only your second post, so no biggie, but just so you know....
There was an instance on Gear Swap a few weeks ago where someone offered more than the selling price to try and trump someone who had already said they'd take the item for the asking price. Things didn't turn out well, and there was a lot of discussion on the ethics of the matter.
While there aren't any hard rules against this, its generally frowned upon to offer more money if the item has already been sold or the sale is pending.
And I agree with John that since he already struck a deal with someone, at this point it should be honored.
So, just a friendly heads up with Gear Swap! :)