You will find different economies by the product, the two I listed are specifically ones that tend to be available for well below what you are saying. For instance, a new steel bookcase from Hon or Sandusky is $400-$800 depending on dimensions and you undergo a lot of stress dealing with shipping contractors probably damaging it. If you roll up in a pickup, or with a trailer or moving truck you can get them yourself for $25-50 in my metro area. Training tables are a similar affair, too heavy and awkward to sell widely but universally available and great to repurpose for shop use. Someone else mentioned cubicles, those tend to be way sub-priced even from name brands like Herman Miller because they are "out of style" -- if you have a large space it is a bargain entry for a high end office set.

If you want something desirable to a wide swath of people like a Herman Miller Aeron chair, yeah expect to pay for it. But if you are flexible, you can find deals. I got Steelcase Leaps for my workspaces when they were not well known for $200 and think they are a better chair than the more recognizable Herman Miller ones..

Auctions are great but they are by nature hit or miss. Preview access is sometimes limited so it's more of a gamble than even a used retail purchase and you may need to take and figure out what to do with a bunch of something when you wanted a smaller quantity.