With the 424 guy, ask him to demonstrate that the unit works in all the various modes and that he's got a pair of headphones and an amp and speakers to play it through so that you can hear and see if it sounds decent. Ask him to have a tape of some stuff he recorded on it. That way you can see if all the channels work. Also ask him to plug in a microphone to each of the mixer channels to prove that all the mixer channels work and that there's no excessive scratchy noises when the knobs are turned. If it works as he claims, he should have no problems giving you a quick demonstration of all those demonstrations.
With the double cassette deck, same deal as far as a basic demonstration by the owner to see that it plays, rewinds, fast forward the tape on both cassette wells and will output a signal from the headphone jack and line out jacks into an amp and speakers. You can also ask him to show you that it can record properly. And for that, bring along a blank type II cassette, TDK SA90 or a Maxell XLII 90. And those tapes are the same for usage in the 424 Portastudio.
With the micro-verb, similar deal to ask the seller to demonstrate that the unit will pass a signal through it and run through the different effect presets to see that they work and play with the level and mix knobs to see that they're not scratchy sounding and can blend between a dry sound to a wet sound with the effect on.
Most honest sellers won't mind demonstrating for you. If they do, take that as a sign that they're trying to hide something and move on. Its also OK to tell tell them that you're new to the recording process and is why you want them to do the demonstration to prove they work. Tell them you're still in school and on a very tight budget and can't afford to start repairing this stuff and that its got to be in full working condition. Most people are honest and if you're honest with them.
Also, about what A Reel Person just posted, that's pretty good advice he gave you and something you might want to think about doing. It would make for a more interesting classroom demonstration to do a live mix in the classroom of your recording and really demonstrate to your teacher that you've leaned something. It might inspire other kids in your class to look into doing something like that too!
Also, all 3 of those ads have been posted at least 4 days ago or more and haven't sold yet, so don't panic that if you don't jump on any of those deals that someone else will snatch them up before you. Take a few days and hunt around some more. Maybe a 488 will come along at a good price too. Ya never know.