Resources for e-commerce: http://www.heppnetz.de/projects/goodrelations/ https://www.access-diva.com/dm2.html Sales and Purchase https://www.vertabelo.com/blog/technical-articles/a-simple-online-store-model-sell-ship-and-get-paid https://dba.stackexchange.com/questions/105915/modeling-invoices-and-orders