Price guides are essentially worthless. The value of an item is determined by
supply and demand. Both factors are subject to
fluctuations which may vary a lot, especially by special interest items. Take collector's dolls, cups or plates for example. If you don't have a really old one from a well-known brand (vintage / from the last century) all things are worth today only a fraction of what was paid then. The easiest way to guesstimate the value of an item is to go to
ebay and take a look at the
sold items and the prices that were paid. That will give you a hint on the "real" value as of today. Do not look at the prices of ongoing auctions, they tell you only what
someone wants to have. If he / she gets the desired price is another issue. If there is no-one paying that price your offer is pointless.
Of course, if you have a really rare item that is not found on ebay at all, the procedure above is futile. Unfortunately. I'll have to make up my mind on what to do in such a case. Maybe looking for similar items would be a start.
There are whole collector's universes that have collapsed during the last decade because interest is fading, and I am afraid plamos are about to see the same fate soon. There are less and less people interested in model building, meaning the number of the ones willing to pay collector's prices is shrinking. Trends might be resurrected but as with all things to come it is impossible to predict the future.
I am an avid action figure collector (mostly Star Wars and Stan Winston creations from NECA). I remember the action figure digest guides from the late 1990s. Every price guide has one major disadvantage: Once published they are already outdated because they reflect the moment the info was collected, even if was six+ months ago. In addition, mostly it remains uncertain how the price info was determined. The only good thing is you may keep it as a check list to see any gap in your collection. Back in the late 1990s you could get original Vintage Kenner figures for pennies of a dollar. It was then when people realized that the figures became more and more scarce, especially carded ones, which turned them into collector's items. The prices went through the roof around the 2000s (and I am glad I got all Vintage figures I wanted at that time

) and led to the situation we have today. However, most colectors are in their 40s+ now. They are the core of the collector base that knows to cherish these things and they are the ones who might pay premium prices for certain items (vinyl Jawa, blue Boba, rocket firing Boba, blue Snaggletooth, the last 19). However, this generation is an "endangered species". No-one lives forever and once the generation that grew up with Kenner toys vanishes the number of buyers will decrease, too. That in turn will inevidently lead to a price decline, the collector's market will collapse. Sure, it is "Star Wars", and that will be with us forever, and right you are. Nevertheless, what do you think, how many people who are growing up in the age of smartphones and online media will be interested in some old and dated action figures? They will most certainly call it rubbish and move on. Thankfully I won't be around any more when that happens.
(EDIT: I have edited the post when I came back from work, so if you have already read it please do it again. Thanks. 
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