Hi Adrian,
The limit creation of Microsoft Accounts is designed to deter hackers from creating multiple email addresses and use it for 'spam', or to 'reserve' names that aren't going to be used at all. Currently, there is a creation limit of 3 Microsoft Accounts, per Day, per IP Address.
If you want to file a request for a specific IP addresses to be whitelisted from this creation limit, you can submit it here. Prepare some info about your computer store at hand so that your request gets approved.
Hope this helps.