This.
If the OP had said he would discuss the price when he came to pick it up fair enough, but you verbally (or actually technically in writing) struck a deal with him. It doesn't seem very "nice" to then say "ya know what, I think I need 5 or 6 percent off because people on the internets gave me info on how to save more cash".
I didn't strike a deal at all. They responded to an automated 'pricematch' service on their website.