What he means is that you can't determine the value of something by looking at how much people are asking for it on the GTN. Rather, the value is what other people are prepared to pay for it.
100% agree. You see it all the time on the TV show Pawn Stars on the History Channel. Customer walks in demanding $X for item Y because he saw it online selling for $X. The first question out of the shop's staff is, "did you see someone actually pay that much?"
Since there's no way to see past sales though, it's often impossible to find this out. With high-volume items such as materials or companion gifts this is less of a problem, since low-priced items are gobbled up pretty fast (the popular types anyway; I dare you to try selling luxury fabrics at a significant profit), leaving the market in somewhat of a stable state.
what you can do though is if you are willing to spend the time, look at the range of pricing and make the assumption that the average of that pricing is the high end of what most players are willing to pay.
Or you can use your own set of values and assume most players are like you.
Either way, you adjust your assumptions as you get more data.
But with rarer items like schematics you often have to make wild guesses at how much it might possibly be worth.
One nice thing about the GTM is that while there is a deposit to post a sale, the GTM does not get a cut unless you actually sell the item. So if you have an item that is currently not available on the GTM, shoot for the moon post it at a very price, worst case is a day later you get it back unsold and you post it for 1000 (or whatever) credits less
until it sells. So long as you have patience, you can figure out what the real value of an item is.