How to get lifetime sales value and orders of a website via soap API v2?
There isn't a ready to go method for this but you could use the invoice.list and the creditmemo.list API's to calculate the sales.
You can use a past date like 01-01-1970 up to todays date to filter on all invoices or creditmemos, retrieving all of them and using the
grand_total records to retrieve the lifetime sales.
A small warning, the API will get quite slow when retrieving that much info. You might want to think about creating your own WSDL in Magento to retrieve the data. There is some nice documentation on this topic by Inchoo and one post on the Magento Board