Identifying molds can be tough. Some manufacturers use the bullet diameter and weight to identify the mold; some use a diameter and sequence of numbers; others simply assign serial numbers. Sometimes “extra” numbers are stamped onto molds. Sometime mold numbers change.
The “single cavity,” "double cavity” part is simple: it is simply the number of bullets that can be simultaneously cast. But not all molds with two cavities, for example, are double cavity molds: sometimes two different bullets can be cast from a single mold.
In all cases, the manufacturer has to be part of the description. Besides that, include all the numbers stamped onto the mold and make a list. I think most of them can be identified from that. It may seem that pictures are a needless hassle but, if you want to sell them on the web, they help prospective buyers see what condition they're in.
I wish there was one spot you could use as a reference. I suspect that a collection of that size holds some old, discontinued molds that would be hard to find on the web. Some of them might be very desirable, some of them virtually useless. On line auctions might be the best way to get what they're worth out of them.
If he's got that many molds, I'd bet he's got lots of other reloading equipment and supplies lying about. Can you run down a local friend of his who can help catalog it all?
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