Okay, this is going to sound like a silly question, but here's what I wanna know.

When a character goes from one rank in their career to the next, are they still able to purchase advancements from earlier ranks, or are they restricted to their current rank?

Like for example: If an Assassin makes Shadesman, and wants the Ambidextrous Talent from Sell-Steel, is it still possible to buy it?

Short answer....yes....every list remains open at the same cost....

I like the short answer, but if there's a long answer, I wouldn't mind elaboration.

I asked mainly because I couldn't figure out from the core rulebook if it went without saying or not.

Long answer, almost the same as the short answer. The only real difference is "Alternate Ranks". If you take one of these, the normal career rank that you trade in for it is no longer available to you normally ... though you can purchase advancements from the rank you gave up as Elite Advances.