One trick I have done is to buy these panels for $35 that are double sided and then make them into single sided. In wanting any used walls you have to consider possibly the gas it takes to go get them. From what I am seeing presently in lumber is so nice and inexpensive that it might be better to go with new. It depends on wether you are building a disposable haunt or want it to last for several years.
Some of the walls we use have been reclaimed and are going into their 25th year of service. Pretty creepy. Lumber just 2 or 3 years ago was not cheap and so there won't likely be cheap ones available for resale so fresh.
The other factor is wether you value your time or not. If you pay to have them built and primed or as we do here prime first then assemble, you might have as much as $50 into each one. Plus as you didn't put every screw in yourself, and it is pretty boring pointless work you will find where your helpers din't put screws in like you would have or lined them all up just right. This makes $35 sound pretty good.
Another fabulous post from the U.S.Department of Wild Imaginings, now in spectaclar stereo, sponsored by the Adhesives and Sealants Council, suggesting ways to stick things together since the 1800s. Not fabulous in a gay way. Your results may vary. Illinois residents add 8% sales tax. These posts have been made by professional post makers, do not try this type of posting on your own without extensive training, lovely assistants and a trusty clown horn.