I only we had machines which could perform these difficult lookups for us, and directories of information that could be queried to determine which taxes apply for a given address, along with some way for these machines to communicate so that different tax jurisdictions could keep the sales tax information current... Oh, that's right, we do.
The claim that determining the applicable sales tax for any given shipping/billing address is difficult is complete BS.
One small example: many jurisdictions tax different categories of items at different rates. This means that you'd have to start off by knowing those rules and categorizing everything accordingly. After that, you have tons of edge cases to work out, e.g., what happens if I group two different categories of items into a special package? What percentage of the package is taxed at what rate? Or, what if different states categorize the same item in different ways? Or, what happens when you provide a service to sellers and then need the sellers to accurately provide all of this categorization for every jurisdiction?
Of all companies, Amazon probably has the best chance of getting this all figured out if they really had to. However, calling it difficult is by no means BS.
Adding on to your excellent point, which tax configuration do you use for calculating the tax? If the affiliate is what causes nexus, do you use the affiliates local tax rates for calculating tax obligation or do you use the customer's local tax rates for calculating tax obligation? It seems absurd to me that I would pay more in taxes if I click on a link from Alice's website than I would if I were linked to the same product on Bob's website.
The claim that determining the applicable sales tax for any given shipping/billing address is difficult is complete BS.