Worth the money?

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If they're all full, you can drop $100 on one box and it would be worth it. So yea, $100 on those boxes is worth it.
Depends on two things:
1. Do you enjoy looking through that stuff?
2. If you're married....is it OK buying and filling your place with 100,000 baseball cards (my wife might shoot me...or help me sort it all out).

If yes to both, then go for it!

Side note: I'm guessing there's plenty of good cards for TTMs in there too!
Doesn't sound very promising to me. "mostly commons and many duplicates" would keep me away. Look at the years/brands of the cards. The seller has basically told you there's nothing worth anything here. But, if you have a plan for those kinds of cards go for it.
I don't like writing on my storage boxes, unless I put it there, and then it's pencil. However, the 25 empty boxes new, you'd probably spend close to if not more than $100 right there. I want to say the 3200's sell for $4 and the 5Ks sell for around $5 each. Now consider the boxes are used and written on, but if that doesn't matter, it's probably worth it if you can use the boxes.

Going through the boxes, especially if the seller is spot on with the description, could be tedious. However, there is a sliver of a chance there are some gems hiding within as well.
At the flea markets I've been to, you could have taken that lot and sold 1000 cards for $5 a pop and make a nice profit.