UPDATE: I took the Marlin to my local gun shop and talked to a guy who, while not a gunsmith, knows his stuff. He's seen more than his fair share of lever guns, but he's more of a shotgun guy. Anyway, he took it in the back, took it apart and examined everything. He said he could not find anything broken or obvious. There is a timing problem, but he couldn't quickly determine what was causing it.
What's happening, and he spent many more words that I quickly forgot in telling me this, but it's feeding two rounds at once. That's why it jams, because there's another round underneath it pushing on the top round.
I've followed the links above and read a lot of various things in various forums, but I'm just not comfortable really doing anything with this gun, particularly since the guy I bought it from will gladly return my money at any time, provided I don't eff it up by trying to fix it!
My next step is to send it to Marlin, to the tune of $45 plus any parts (that's my understanding anyway), or to find a gunsmith locally who can tell me over the phone that they would fix it for less than that (and I'd avoid shipping). The guy today gave me a couple names but I haven't talked to them. Any thoughts or advice at this point?