Don: welcome to the web site (your 7th post). Before looking to sell these stamps, you may want to check to see if you have stamps of any value, more than the minimum Scott catalog price. Most larger libraries will have a selection of more or less current Scott catalogs. You're not looking for the exact current value, but more to get a picture of value for the past few years. Bear in mind that even pre-WW I stamps can be pretty common and of minimal value. It may be that your stamps are mostly common values and thus Mystic wasn't interested. Companies that sell in bulk, like the two you name, also like to spice their lots up some with higher value items.
Your 1950s plate blocks are best used as postage, especially the 3 centers. I have a bunch of them
inherited from my father when he passed away in the early 1990s. Same for 1st day covers; those from that time frame are generally of minimal value. Too many people saved this stuff during the '50s, thinking they would make big bucks when they sold.
You may end up just giving the stuff away to an appropriate charity. I live in metro Denver, home of the Rocky Mountain Philatelic Library. I donate a lot of my common stuff, IF in good condition, to the Library for their youth program. The tax write-off is modest; just because a stamp catalogs, in Scott, the minimum 25 cents doesn't mean it is worth 25 cents.