Just offering my tuppence here.
The Mew cartridges on Ebay are simply .sav dumps on to retail cartridges of FireRed. The same sellers on Ebay offer them every few weeks with various prices ranging from "$200-500".
The OT is Red, National Dex (including Celebi) is obtained, all badges. 4* Trainer card if memory serves right. Game time is usually 100+ but it depends on how often they've had it happen on a GBA/SP. Game time though doesn't really show anything in particular.
The Mews are clones yes but how do you explain the same Pokémon traded from customers across multiple cartridges? (Unless they're smart enough to delete them).
I'm not suggesting that some of them aren't real by any means but the same sellers list these cartridges claiming they received them from former emloyees of Toys R Us. Ebay has an archive, and the fact they've listed (and sold) 10 Fire Red Mystry Mew distribution cartridges in the past year is a pure sign of .sav dumping.
As for the Mews themselves, yes there are only 420 Mews available but with save editing possible, editing the Mews to make them 'unique' isn't really difficult. If someone does in fact dump the save, Legal.exe them and it shows they're hacks, who are they going to complain to? They're hardly going to complain to Ebay about a Not For Resale Item.