Expeditious Retreat is having a
The sale is because they are moving, and their warehouse is their home.
They are not going out of business, nor are they pulling out of 4e. These are all physical books, not PDFs.
I have never looked at their 4e material, but I can tell you that much of their 3.X and True20 material is first rate. Unlike many 3PP they have been very successful with 4e, which speaks for their 4e material being of similar quality.
They do not have much 3.X material left, but...
Of the Magical Society books only two are on sale - A Magical Society: Beast Builder and A Magical Society: Ecology and Culture are going for $2 each. The latter is quite useful for any game, being about world building, with little to tie it to the 3.X architecture.
Their Monster Geographica line is also $1 each - a series of monster compendiums broken down by terrain, then by CR inside the books. The most useful format that I have found for critter books.
200 monsters per book, at $1 per book. If I didn't already have them as PDFs then I would be ordering them. I may anyway.
Their shipping rates are... odd. They charge $2 a pound on shipping, then reimburse any money left over from the actual shipping costs. (Did I mention that their warehouse is also their home? We are talking about a small company....)
The Auld Grump