depends on your honesty, integrity and how you'd like your reputation to be on here,
or do you wanna be known for "walmart" style bait and switch tactics
if you gave me a different price than what was advertised i'd say FU and file a paypal case (if i'd already paid) and give you a lecture on how morally wrong such a thing would be.
i would also probably start a thread so everyone could know not to ever buy anything from you.
If you quote someone a price, then change it to a higher price when the sale is on...... I would say you are running a scam! Dealt with that type many times as a contractor.