Hello, here's another thought about inducing customers to sign up - you can offer then a coupon for your Free product for signing up. You can limit your coupon to a specific item when setting up your coupon.
This will accomplish several things IMO - you can validate that the email is "real", and give you the opportunity to contact them in the future, and it will remove the freeloaders that have no intention of becoming customers but just can't resist a freebie.
You can monitor your coupon redemption and send them a thank you for "looking" and all the additional good marketing followup that will garner you additional sales.
Just a suggestion based on employing continuing marketing contacts.