Duped in this case is a literal word. It means defrauded, fooled or misled. In short, you led the visitor to site A, and you switch them to site B. This is pretty much the core element of a phishing attack. While your intentions are good, I'm sure - it is going to be hard for that user to be certain of that. Give them a reason to doubt security and the probability that they will bail on the transaction should go up...
Product synchronization should be an obvious issue here. If product A has index of 2 on Site A but 298 on Site B how will you know which URL to redirect to ?? This is not as simple as it sounds, especially once you add in options and option values...
David