Wert knows me well. Nick: Welcome.
Just as quick background : I only collect Newfoundland items and thus my comments or viewpoints are narrow and may be helpful or not. I have a large collection, since about 1968 and I am very active buying stamps and covers from auction houses, stamps shows (old school but still good finds--this weekend in LA at SESCAL), and from
ebay and other online sites.
In my viewpoint, for the "market price" you would be best to buy Walsh's Newfoundland Specialized Stamp Catalogue (NSSC). There are old paper copies on the web or buy direct in a PDF (the 2014 9th edition).
This is not only far superior for details and varieties than Scott or Unitrade--it also lists observed prices paid for items. Again--unlike the "Catalog Price" in Unitrade or Scott, Walsh and his company updates for actual prices they have seen. So its a great guide on sort of the maximum you would want to pay. I regularly use it to both overpay (making fun of myself) but also get huge deals with others who naively use the 40-50% of Scott value approach.
I admit that I am subjectively biased and don't want to offend anyone, but I love NSSC for this (though they have a different number sequence). Let me give you an example : let's say you see a cover with the 3 cent blue Queen Victoria (Unitrade #34) for sale at a show for $40. or a dealer has one. or its on
ebay. Scott/Unitrade lists the used stamp at $20 VF and the cover at $80 (I'm guessing, as I don't have the cover price with me). Walsh's NSSC has the used stamp at $20, and the cover at $160...and on the cover--they actually have a census of the known covers in existence of 115 in the world. So you can safely pay up to $160 for the cover and know that you have one of only 115 known covers.
Good luck