One pitfall you could run into trying to do it by sales, would be if a customer has purchased material say, it 2017, then returned it. They’re sales would net out to “0”. Then if they have sales in 2018, they would show up as a “new” customer.
The best way to do go about it is going to depend on your source data, and what fields you have available to you. In our case, we’ve got a “Date Created” field on our customer records. We’ve included that in as an available property on our customers in Phocas, so we can simply filter based on the date the customer was created.
Maybe you have this available, or can possibly get it added to Phocas, if it’s available in your source/erp data.