I see that u two dudes both ride old ratbikes...and there is nothing wrong with that. BUT, if he wants to truly save $$$, then a WORN-out "bargain" bike can be just the opposite. They will quickly nickel-and-dime you to death, PLUS you keep missing out on riding, waiting for the UPS truck.
Southern3.0: Parts prices for bikes go WAY up once they are several years old, making a "primo" used bike actually CHEAPER in the long run than a well-worn, "really cheap" one...
Used bikes are only as good as their needs...KTM's, Gas-Gas, and Honda's tend to last the longest. Also, 1990's Yamaha's like yours tend to wear out the tang on the powervalve, making it then contact the piston, so I would be leery of a used YZ powervalve, sight unseen! Otherwise, salvage yards are great for a lot of stuff.