most people i have dealt with in life, both in real life, and on the internet, will work with you if you are upfront and honest. so...why did this go south? did you immediately contact him and ask him if he would be willing to let you back out? or did you say, too bad, I cant afford it now? i think, if someone commited to buy from me, and then came back and said " i am willing to keep my word, but if its possible, i would like to be released from my obligation" I would say " no problem"
but, if I listed an item, and then worked with a guy to get the payment, and then was told, "sorry i cant afford it now, im not keeping my word", i would probably say that person was welching. our word is all we got.

oh and this isnt ar15.coms fault. they have a zillion transactions, thats how it gets done.