Oh -
Some places use in-place Estate Sales - the stuff is sold out of the house it lived in. In other places (at least used to be (I type that a lot 
)) auction houses would gather the stuff from multiple houses, bring them together in one large space, and then sell all of it in a single evening. While I lived in Indpls (hell hole), I picked up a cute little vacuum cleaner for $7.50 - I kept it a primary, then secondary for over 20 years. A nice sofa was $22.00.
The big advantage to thse places is they are in the business of moving stuff, and offer cheap delivery.
Has anyone mentioned IKEA yet? 
I go for the solid wood options, e.g. the Hemnesbookcase instead of the more popular Billybookcase.
That said, we own two Malm bed frames which are particle board, but they have both served us well for over five years and still look like new.
For some, “good furniture” does not require assembly, and, when wood is called for, expect hardwood (hickory is always good - why so much furniture comes/came from the Carolinas).
But, since the trend is to flat-pack:
If you get real wood - even softwood - and use glue (white Elmers will work; yellow carpenter’s is preferred), you can get a sturdy product - the glue attaches to fibers which run throughout the board.
Particle board cannot be (meaningfully) glued - the glue will stick to the nearest particle - not the board.
I have a flat-pack desk from a real office supply store in 1982 - it is 4x the quality of the stuff in the discount stores, even though the top is particle - the drawers are rel wood, and the slides are metal and on the sides, not the cheap-ass bottom runners.
Yes, the real office supply store was run out of business by Office Depot.
Gresham’s Law. Gresham never knew how universal his observation was.
Fark had a list months back of “10 Things to Never Buy New” - and you’d better believe Ikea was on it - between students selling off stuff at end of year and plain old “got a raise”, the re-sell value of the stuff is near 0.
Craigslist is great, so are thrift stores. I suggest you look for the odd thrift stores, I’ve had better luck at a local Christian thrift store than at a chain thrift like Goodwill or Savers. You might find something in Etsy, but try to buy local. Another idea is to check with your state’s prison system. Some states have a furniture or cabinetry training program for prisoners and you can get some lovely stuff from them for reasonable prices.
Resale and antique shops often have good stuff.
We did an IKEA Hemnes bedroom set for my daughter - she’s fourteen and won’t need it forever.
It won’t be an heirloom, but its going to last a LOT longer than she needs it. Its very sturdy.
My own bedroom set is Ethan Allen. Its nice, its held up well.
Couches we abuse around here - buy 'em cheap abuse the hell out of them, replace them in six years.
I usually get better deals when shopping at divorce sales. Things are really priced to move.
Reported.