I prefer to buy from a grower or hybridizer. I go to the auction for things I cannot find other places.
It really depends on what you want. I know there are all kinds of folks on NGA and some want only the latest and greatest cultivars. Others are looking for some that hit their want list or a certain look, size, shape, or color. We have purchased daylilies from all over the USA. Facebook and the LA for us is specialty purchases at this time of our collecting. We have over the years purchased from several people on the auction both plants and seeds. Our experience has been good with only a couple of issues from 40 different sellers.
I typically start out wanting a specific daylily or two and then add to that list based on the seller that I am buying from.
If you are just collecting several nice cultivars to get started or fill some beds there are some gardens with great prices and and selections. On the auction you will typically pay a little more for the plant and shipping. So it is good for sellers but bargain hunters must have self control.
For seeds, it is the LA for certain.