From what you have said you had every right to first come first served.
If dog had given you a deposit then I would say you had a responsibility to hold it.
One thing I might have done differently was to email dog before you met this other guy and given him the opportunity to bring his car down the same day and allow you to choose between the two, but that is just me.
I am sure opinions on this will vary widely.
Still he should not be bad mouthing you if he is.