Please note that prices on Pokmon collections change quickly
For example, if you control The Ozolith ("Whenever a creature you control leaves the battlefield, if it had counters on it, put those counters on The Ozolith.") when Sin's last ability resolves, you'll put the appropriate number of each kind of counter onto both The Ozolith and the target creature
The proxy does not need to appear inside the store, but it should not conflict dramatically with the account, selected store, and normal customer location
And finally, that the mana base just isn't as premium as most the spells it supports
Popular sets like Mega Symphonia and Mega Dream EX demonstrate this clearly the price gap seals the deal: For a deeper comparison, see our full guide to Japanese vs English Pokemon cards