I wanted to share some of the cost effective ways I know of to buy building materials for your BRRR or rental.

* I have a lot of luck with Facebook Marketplace. Toilets are generally free if you don't mind pink. The heavy items are free too so if you want a big credenza and have the help the price is right.

*Hibid.com is online local auctions that you can bit on and then drive to pick up your purchases. This is an outgrowth of in person auctions and so there is no standardization like you get with eBay - Know when the pickup is and that you will be available and also be mindful of payment methods. They will vary between auctions. Further, Buyer beware - some of the stuff will not work and you have to plan accordingly. I bought a salamander heater once and it came with the cutest little mouse I've ever seen who decided he liked my car. 

* Woot.com is Amazon's bargain basement and there are deals to be had - like a 250 basement dehumidifier. The deals are all "while they last" and some of the deals aren't deals at all, so know know what you are getting and what it's worth.

Hope this helps