Well as soon as someone opens one in pack and sells it back, it becomes available in the merchant. Last week Euk opened and sold some RoS starter foils (some Guardians and a Orophin i think), but now i see they are out of stock again. Wonder if someone bought them for like 1500 (or in Orophin's case more like 4000, cause he got 3000 for selling him) or what.
Its very hard to open one in a pack though, i spent 2000+ gold yesterday trying to get one in Hunters (though its a large set and its harder to get lucky there than in RoS) and got nothing. At least i decreased some of the regular rares' prices.
I got this impression though, that every rare gets a little bit more expensive every day or so, even when noone buys it, otherwise cant explain Brute's price for example.