Whoever bought a Scout from the Black Market deck, just to Masquerade it over, I will buy them a drink.
Edge case where it makes sense: It's a bad board - no extra buys (except possibly in the BM deck), only Highways for $5. You just so happened to draw $4 with four highways, a Watchtower and Black Market (obviously some on deck, drawn from the Highway) and thanks to deck tracking, you know your next hand will be mostly Duchies and Provinces, but there is also your Masquerade coming amongst the 5 cards, and it'd cause a reshuffle, if you can play it safely. So Black Market turns up Scout, you buy it for $0 and top deck it with Watchtower, then buy a Province, next turn, you Masquerade over Scout.
...Let's see if people can't put holes into this scenario.