langhorn wrote:
Iwas the original poster of this thread, so I thought I'd report how successful I was (not very), and make a suggestion on how to help people in a similar position.
I bought an obsolete (1991) set of Scott catalogues from a library sale. Although out of date, these were a big help because my stamps were old, too. I recommend that people buy a used set if they have old stamps to sell. If nothing else, they're interesting.
I made about $20 from my auctions -- not worth the time required to scan and post, but better than nothing. The majority of that $20 ($12.30) was one item -- an envelope with three old Italian stamps. I deduce from this, that collectors prefer stamps on the envelope, not soaked off. Or that's my theory -- you all can tell me if I'm correct.
Naive people like me need some generalizations, that they can use to decide what they have. For instance, one person told me "Nearly all post-WWII US stamps are valueless, except as postage." (I assume that means that if they're cancelled, they're not even worth that). Of course there are exceptions to every generalization, but a few guidelines like that could save we beginners a lot of time.

Obsolete catalogs are my first suggestion when someone really wants to know something about their stamps, other than just 'what can I sell it for?" They're typically cheap ($1-5 each for 10 year old issues) and have very close to everything a beginner want to know about the stamps.
Rule of thumb 1: Stamps on covers are worth more than stamps off covers. That doesn't say it'll be worth much, just that on cover examples are almost always harder to find than off cover examples. Exceptions are for some 'created collectible' type covers, which will often be worth less than the stamp by itself.
Rule of thumb 2: Unused stamps are worth more than used stamps. (Note these don't compare unused stamps with covers.) In general, most stamps are used for paying postage, so the used ones outnumber the new ones. There are exceptions, typically when a stamp that meets a specific rate has a very short life, such as a rate that is only in effect for a few days after the release of the stamp.
Rule of thumb 3: If the stamp is from a big country, and matches the current (to the stamp) first class postage, it won't be worth a lot. Even stamps issued in the 1860s were often issued in huge quantities and therefore saved in huge quantities.
Rule of thumb 4: Collectors almost always pay more for their stamps than they will sell for. Even when the duration between buying and selling is quite long, decades or more. This is how dealers stay in business... 
It's worth noting that the 'Nearly all post-WWII US stamps are valueless, except as postage.' is totally incorrect. It's just that they're worth *more* as postage than as collectible stamps. old postage lots typically sell for 70-85% of face value in the retail market and 50-70% of face to dealers. Even 50% of face is far from 'valueless' when you're working with all those sheets aunt Ida saved...
Online Stamp Collections 