Generally speaking, the public cares not a jot where their product is coming from, and most of them will take the risk of the Revenue slapping duty on them because it happens so infrequently.
With regard to the bill from the Revenue, you would get only one: import duty.
I have reported those sites to the Revenue in the hope that their packages will get stopped at Customs.
With regard to selling on eBay, or anywhere else online, there is nothing in the Ts and Cs from S2 about selling restrictions. Now, I understand about the "spirit of the agreement" (legal term), but if we, as customers of S2, want eBay selling to be prohibited, then the Ts and Cs need to state that.
Your statement that people are out to make a fast buck? Of course they are! It's called business! Those of us who saw the potential in Vinylux got on with it and promoted it, and hell, I'm bloody glad I did! My sales of Vinylux pretty much stopped overnight when the US sites started shipping to the UK, so I'm glad I got in when I did.
Ask yourself this.
Who, in the supply chain are the only ones not adversely affected by the these US sites selling to the UK?
Is it you, or me? No, we are because we're losing retail revenue to them
Is it S2? Nope; they too are losing sales because of it
Is it the general public? Well, yes, because they can get what they want, when they want it, at a low price.
Is it CND? Well, yes it is. Clearly their only concern is to sell as much product as quickly as possible before other manufacturers reverse engineer it.