What PODs would prevent Russia from selling Alaska?
No Seward as Sec. State might prevent the OTL purchase.....other than that, it'd be tough to pull off; Alaska was extremely tough to keep a real hold on, and it wasn't even explored all that much until the late 19th and early 20th Centuries. The most likely result of no purchase, IMHO, is Alaska eventually becoming an independent nation once enough foreigners move there for the Gold Rush that will almost surely occur at some point.
Of course, you *could* potentially make Alaska a satellite state, or even make it a "commonwealth" of sorts, much like say, American Samoa in the South Pacific is to America in the real world.....with the right cards. But one thing's for sure; with a POD that late, Alaska is NOT going to (plausibly) become a permanent integrated part of Russia, due to various logistical problems.