The overriding consideration when you are on the fence about backing or not backing is that you can always later back an unbacked poster but you can't really ever "unback" a backed poster, so why not live with it unbacked and see how you feel down the line?
As to people accurately describing condition, I used to buy online all the time from random eBay sellers, but I have almost completely quit (look at my eBay feedback: user name emovieposter.com) and the number one reason was how many people flat out lied on the condition (I would get items with tons of pinholes or lots of waterstains, unmentioned and graded as "very good" and there would be a fuzzy photo). The number two problem was always poor packaging.
Ask around, and if a few different people say "I had a problem with X seller or auction" then that is probably someone you don't want to deal with, or you may well be the next person saying it! And the vast majority of people won't comment publicly about negative experiences, so ask in private emails (the best people to ask are long time collectors who have large collections, because they have probably dealt with just about everyone.
For example, I have never bought from Dale Dilts (I have sold to him), but I have never ever heard even ONE slightly negative comment about him, and I have heard a ton of positive ones. Isn't this the kind of seller you should be seeking out?
Bruce