Anonymous Freak has pretty much covered it. "Abandonware" has no legal status - in the eyes of the law, its piracy. Having a dodgy copy of Pac-Man for the Apple II is still very much the exact same thing as pirating say, Diablo III.

Unless the developer has made the software available for free since it was sold (as in the case of System 7.5.3 and Cliff Johnson's games), by downloading it, getting a serial, etc when you don't have a license for it, you are still violating the terms of the EULA just as you would be for current software. Yes, you do have to buy it on eBay, or in a thrift shop, or whatever.