Sorry for taking so long to weigh in. Silly time zone difference!
The Marlin 60 is a great rifle! Unless you have to work on it. For a $100 it's really hard to go wrong. I have about 7 different types of the "Marlin 60" (Glenfield's, Marlin's, Montgomery Ward, Coast to Coast). Many times if there is an issue with the $100 rifles and shotguns, it can cost more than the firearm is worth to repair. You figure it might need $34 in parts, $11 for shipping of the parts, and $60 in labor, now you're above what you paid for it (an actual repair). That's for something relatively "minor".
That being said, it's highly unlikely that you'll find a Marlin 60 that needs repaired if it was treated well. If I were looking at the gun, the first thing I would check are the bore, muzzle and chamber. In that order. If the bore is crap, walk away from it. If the muzzle is tore up, sell it to me.

But unless you have your own lathe to re-crown a muzzle, it's going to cost you too much. The chamber is the next place to look. As others have mentioned, you're going to need to look for signs of cleaning rod/brush damage. You also need to look for firing pin damage.
This is a rimfire weapon. Meaning, if little Sally or Brian sat around dry-firing it, you could have two problems. First, there might be a dent that corresponds to where the firing pin strikes the cartridge. On a Marlin 60, that's going to be at the bottom of the chamber. The second issue is going to be with a bent or broken firing pin. The only good way to check that out is to remove the bolt from the receiver and flip it over. You're looking to see if the firing pin is straight, and if the tip is chipped or deformed. Generally, if there's not a dent in the chamber, then the firing pin should be fine.
I also have a different take on the $20 scope. What better use for a $20 scope is there than a $100 .22 rifle? I'd look at what brand of scope it is, and what condition it's in. One check that I use (albeit very low tech), is to remove the scope from the rings and shake it next to my ear. If you hear something moving, it's trash. As nelson96 mentioned, it's not going to be very useful if you're pushing the limits of a .22lr. It might fog up, it will suck in low light, and doesn't give you a very wide field of view. BUT...... For daylight use of a .22 on a range? Perfect.