If all you have is 20 unique issues, then this is grossly misrepresented. I can't imagine those types being all in G to G/VG unless there is overall water damage. But either way, a 50% or more refund seems to be in order.
My impression was about 50-70 individual issues where only a 25% refund might be in order.
BTW here's my point of reference. I have a warehouse seller who sells me long boxes all the time (see my Golden Key sales thread). He sends them to me first. I review and make an offer which so far he has accepted each time. Even then I get disappointed because there's only so much I can do with 25 cent Harvey's. But no way in he11 am I going to pay before I get to see them. It helps that we've been doing this for 3 years now so we have mutual trust, but I still wouldn't pay first.
That said, I feel for you, as having crappy books hanging around the house is a bear. I say eat the loss and move on, it's not worth the hassle other than a paypal dispute which you're following.