I am assuming this is in reference to this thread?
As far as legality goes the laws vary from state-to-state so it will be difficult to give you advice. Even then most states don't have many laws governing this.
Your biggest issues will come from the the Payment Card Industry and your merchant account provider. It is against Visa and MasterCard guidelines to use a traditional retail merchant account for Internet orders. It is also against Visa and MasterCard rules to use non-ECI compliant means (e.g. a credit card terminal) to process Internet orders. All Internet orders must declare themselves for each and every transaction. Credit card terminals cannot do that.
Additionally, to accept online orders and then enter them into a credit card terminal will cause you to violate PCI rules that state that prevent you from storing CVV information. You also force yourself to enact full PCI compliance since you will be forced to store credit card information which is difficult to do and a royal pain the butt.
Using your non-swipe merchant account for Internet orders, even for the same business, could result in anything from a warning, to a large fine, to the merchant account being closed and the business being added to the Match File. Once you're on the Match File you are blacklisted and cannot get a true merchant account again.
Your best course of action is to call your merchant account provider and discuss this with them. They're the best people to speak to.
FYI, I worked in the industry for 6 years. I worked directly with the acquiring banks and both their underwriting and security teams. I know it's tempting to use the retail account but it's against the guidelines set forth by Visa and MasterCard and will only result in big problems down the road. Get the second account and use a payment gateway to process the payments. Doing anything else would be an necessary business risk.