It's not impossible at all! It's just takes time and research.
There are known, authentic examples of all the items you have that have sold in the last 10 years. It's a matter of tracking down those records, and seeing if your props conform to known examples. There are all sorts of Star Wars items that have hit the open market, and thanks to the internet, there are far, far more photos of the various screen used suits out there. The resources available to an appraiser today are incredible compared to even 10 years ago!
It just takes time to wade through all that, plus researching the market. When something goes up for auction, an auctioneer uses their experience and judgement to come up with an auction estimate, same with an appraiser. The appraiser just has to show more of their work, or should anyway. It's coming up with the opinion of value that can be the tough part.
For example, Profiles in History sold a Bapty Stormtrooper Blaster in 2010. This blaster had accurate replica parts added to bring the blaster back to its screen-used appearance, and that was stated in the auction description. Jason DeBored did a great write up of the blasters on his blog:
http://www.originalprop.com/blog/2009/01/09/star-wars-a-new-hope-batpy-stormtrooper-blasters/
The auctioneer gave an estimate of $6,000-8,000, and it ended up selling for $13,000. Was the auctioneer "wrong" in their estimate? Not at all, they gave an opinion of value based on their experience. A few years later, in 2014, Prop Store of London sold a Bapty for $30,000. More or less the same gun, with the same alterations. If I was going to appraise a similar blaster in 2017, I would use those records to show the value increasing over time, and use that and other market data to show how the value of these items is doing, and value the item accordingly.
You're right about the authenticity, there may always be questions, but at some point, authenticity is a matter of probability and faith. Is the letter real and accurate? If it is, the props are likely real. If the props conform to known authentic examples, then they are likely real too, and the letter reinforces that! A savvy collector knows which to give more weight to. A blaster that shows up on Ebay with a story of how an uncle worked at Pinewood Studios in England, and was given the blaster sounds good, but it might fall apart under scrutiny. Here's a hint:The Original Trilogy wasn't filmed at Pinewood. The seller might have mixed up the studios, but it certainly gives pause to a knowledgeable buyer. Then it falls on the blaster itself to show itself as being authentic, and not a recast of a repro sold by a charlatan.
What I'm saying, is that it's not impossible to value them, and while I could throw out a number, I would not be comfortable doing so without more research and verifying the authenticity of said props. Where are you located at? I might be able to recommend someone close to you who might be able to help.
James