The little that I have fooled around with it... If your Sims own a property, and you use the phone or computer to "Check Real Estate", then you can upgrade it from there. The menu shows you the value of upgrades you need to generate more income. Once you start the upgrade process it is much like building a house and is dependent on your family funds.
I have not figured out how to do that from the family's inventory as it never really occurred to me. I did upgrade the Consignment Store and then sold it after one week's worth of income. The new sale price paid for the upgrades, not counting the income, and selling it let my Sims sell at it again.
Tomb-raiding doesn't work through the Consignment Shop, as the items cannot be consigned. I have only been able to sell them from inventory. I do keep some on hand in case one of the kiddo's gets the School Opportunity. They are worth double the amount if sold from inventory if turned into to complete the opportunity.
