You have a great find. This is all prewar American Flyer circa 1940-1941. It's S scale, meaning the cars and locomotive were scaled at 3/16 of an inch to the foot; but O gauge using the 3 rail track as pictured. After the war Flyer converted to S gauge, 2 rail track, while continuing to expand on the S scale line they first introduced right before the war. The 3/16 inch to the foot was designed to more closely represent actual cars and engines than the earlier Flyer pieces sold in the 1920s and most of the 1930s.
The pictures show that the cars and locomotive probably don't require any restoring from a visual standpoint. Electrically the engine may require work but these were very well built, and depending how and where it was stored I'd guess you have better than a 50-50 chance the engine still runs. It's hard to tell where you're located now, but if you're in the So Cal area there are plenty of folks around that have the expertise to work on the engine. There is an S gauge club in the San Diego area, and one in the LA area which is the club I belong to. If you're in another part of the country look for a local S gauge club.
As to value. Individually, and saying the pieces are in excellent (c7) condition, or better, Doyle's Standard Catalog of American Flyer Trains value these as:
494 Baggage Car $70
495 Coach Cars $70 each
556 Locomotive $175
These prices are NOT, repeat NOT, set in stone, a lot of negotiating goes on and prices vary depending on how bad the buyer wants them, and how bad the seller wants to unload them. The fact that this is a set and includes boxes adds much more value which again is hard to determine, and based on supply and demand. Bottom line boxes will add value.
Track and accessories such as the transformer and uncoupler usually have little value compared to the rolling stock. One exception which is your case is when they are in their original boxes, which from the pictures appears to be true. So yes, don’t do any harm to the boxes! Also any documentation such as inspection sheets or set up instructions would make a set more attractive to a buyer/collector.
Do a search for the locomotive and cars on the Bay to see if any were recently sold. That would help to determine their value. Even better if you can find a set that was sold.
If you really want to have a go at running these by all means go for it. Even if you had to spend some to get the engine running you’d have a great set for the cost of repairing the engine.
A note about safety. It’s hard to say whether the old transformer is still safe to use. One problem is that the wire that plugs into the 120Volt AC outlet in the wall is often dried out and brittle with the actual wiring exposed--DO NOT plug a damaged cord into any outlet. This could be also true for internal parts. This is not a big problem since transformers can be picked up at club swap meets for a relatively low price. You can also buy new. These transformers put out 7-15 V AC to run the trains, and 15 V AC for accessories. You can even use other brands such as Lionel as long as they put out the same voltage.
Again the bottom line is you could have a very nice vintage train running for way less than the cost of a new set, if that’s what you'd like to do.