Wow. No easy answers to this one. But here goes...Firstly, The US government came up with a definition that is pretty standard for determining a base line. Antique is anything older than 100 years. Vintage is older than 50 years. Collectible is younger than 50 years. BUT Collectible in it's 'common usage' sense is anything of interest to a individual or group of individuals. Hence an object may be both collectible and vintage or antique. Bowls and dishware are good examples of this.
Cars are a special case, as they are only just reaching the 100 year mark. So the collectors for them have a looser definition of antique.
It also varies from country to country. What is antique in the new world (Americas) is generally considered only vintage or old in Britain, as they have historical items going back many centuries beyond here. So you have to also consider the point of sale.
ART is, as the saying goes, in the eye of the beholder. I look at it this way. If the item is created for, or can be used as, a decorative piece - then it can be called 'art'. Even if it has another function. For example: a plate can be used to carry food, or if particularly pretty, hung on the wall. A tile can be used in a back splash behind the sink or framed and hung on the wall.
The key, I think, with art, is to be up front with it. If it's a print- say so. Limited editions are worth more. Check the history of the item too. You mentioned movie posters. Yes, they can be considered both advertising and art. But most aren't worth a whole lot. If you can get posters of interest to more than one group, they are generally worth more. A depiction of a Classic Car from a historical movie would be an example. You now have car buffs, history buffs, and movie buffs covered. Throw a scantily clad girl in the scene and watch the prices soar! (Says something about our culture, huh?)
Research on each piece is the key. How old, how rare, how desirable, how many different groups of collectors would be interested.
For example, a limited number of "revenge of the Jedi" advertising posters were produced and, although relatively recent, are worth quite a lot. That's because they changed their minds and called the movie "return of the Jedi" (revenge was considered an un-jedi trait.) So not many of those posters got out. Scarcity = more expensive. (By the way they accidentally reversed the colors on the good guy/bad guy sabers too.)
So the more famous the show and the rarer the poster the more expensive it is. (the posters come in different sizes too, with smaller ones being harder to find.)
Recognized individuals' works are worth more, generally speaking. As are scarcity.
With books, it's always been hit and miss I believe. First Editions are generally worth something. A signed copy (verified) is worth more. Books that are long out of print are generally worth something. Books with plates (pictures / drawings /artwork) are usually worth more than those without. It really depends on who is collecting them.
For example, books on the Avero Arrow are pretty collected over in Canada and by historical aviation buffs. So they tend to be worth more - particularly in Canada. Specialty books on, say, producing nylon probably won't have much of a market. A book on New York City tends to be worth more in the New York area than elsewhere. So location is important.
Sadly, there is also the argument as to if a book with plates should be broken up or not. Personally, I say not - I'd rather see the history stay together, to me they are worth much more that way. Realistically, the persons who can pay full price for all the plates are much less than if you sold them separately (generally speaking) because a lot of bidders can then be involved. Individually they are easier to display too. So it depends on your values; is it just money driving you or historical significance?
For retail, try and list all the interest groups that may be involved. Some of your research is good for this too. Mention that the book is about steel making, industrial, historical, limited copies available, considered significant enough to have copies in the Ford Library, and so on. Advertise widely and let the market decide.
Ultimately, any object is only worth what you yourself are willing to pay for it. Anything beyond that (what someone else, whom you have been able to find, is willing to pay) is gravy.
Hope this helps!
--------250Chief---------Primary Vintage and Antique areas of interest:Bells (brass and glass), Mining MemorabiliaOil Lanterns, Fire Dept MemorabiliaRailroad Material, Unique ItemsChromatic Glassware