Once sold on eBay, but nothing like this...method sounds cool, indeed.
Regarding Scaling: What tools do you use to automate?
Seems that the selection of items to sell would be tedious without some way to scan/filter. It is the bulk comparison Amazon to eBay that would be key (presumably for thin competition, higher eBay price range vs Amazon), so really need tools for both, then to match items. Amazon has an api, but I am not aware of tools (but also have not thought of this, so have not looked for them previously).
Also, listing in volume on eBay can be a pain...do you use something like Turbo Lister?
Since tax is charged based on ship to address from Amazon, Amazon will collect tax for 23 states. Been a while since I've sold on eBay...does it now have a way to configure to selectively collect tax by state?
Not all items can be from third party prime shippers - who may not collect tax for any state but their own, but you still have to know what state thiers is for collection purposes from your buyers. In low volumes, that may not be hard to manually deal with, but scaling makes it a challenge. Perhaps you have some tools to help manage this? Maybe part of the tools for above items?
Google Shopping might also be a good place for comparison / sales - have you considered or experience with that?