Following up on the vintage encyclopedia discussion, Nick Storm encountered a potential problem for anyone looking at classic era sets- inaccurate dating.
Sellers on Ebay or Amazon may list a collection based on the initial publication year inside the front cover without noticing the edition year. That means encyclopedias dated from the classic era in a sales listing could actually be from a much later edition. In Nick's case the set of books he purchased were advertised as being from 1919 when, in reality, they were printed in 1939.
The best way to avoid any potentially expensive problems is to politely ask the seller to verify the edition date before buying. Most vendors will be happy to refund the purchase price if there is a problem, but they're unlikely to swallow the shipping cost of a return. Given the size and weight of a set of encyclopedias that can be a significant sum, even with the media mail discount.