Hi, in my case it is handy for slow moving items, particularly cheap items where auction or standard insertion fees could wipe out any profit, and the higher FVF fees do not create much damage. But you must also have auction items as shop inventory items have very little exposure when buyers search eBay.
I just subscribe to the basic eBay store, the traffic reporting via Omniture is ok but nowhere near as good as with Sellathon basic subscription. Even the latter I would only subscribe to as required.
The store is also useful for custom categorisation of all your auctions within your store display and certainly when your active listings climb though a hundred or more it makes your wares more easily searchable by a potential customer. You get limited branding options with your own logo on invoices and listing headers even with the basic subscription.
Ultimately the level of subscription you take must be balanced with your sales volume or the subscription will wipe out any profit.
Regarding tax on earnings there is a point at which you would have to declare the income in the UK. I expect an experienced US based community member could inform you about the IRS's requirements.
