Old books I want on eBay.
Spines repaired with duct tape.
Should I buy them for $29.98 knowing it will probably cost me $500 to get them rebound?

@DunwichType if you're just worried about the acidity ruining the paper you could always remove the duct tape and swap it to cloth book tape. $20 dollar repair that at least kicks the can down the road a bit until you want to get it properly rebound.

I guess it really depends if you want it more as a collectors object or a book to be read.