Really depends on where you're located and what kind of cheap we're talking about. For instance, I buy the overwhelming majority of my equipment used or refurb -- anywhere from Amazon (warehouse deals) to Microcenter / Newegg (open box) to Ebay to sites like PC Server and Parts [0]. I also frequent thrift stores (got a perfectly good Epson Workforce 1100 for $9, just needed ink and a little love).

I will say that for some of it, stuff in the "really-cheap-in-comparison-to-MSRP, but still money" category, buying from a vendor that specializes in refurb/off-lease is a safer bet than a mishmash of Ebay parts just in the event that a given component is bad or unreliable. For instance, I recently put together a dual-Xeon box and the riser board was damaged in some way I couldn't repair; PCSP sent me a new one, problem solved. Would've been fairly costly had that been an Ebay buy (especially from a Chinese vendor, the shipping now! my god). All-in, I ended up with case, power supply, riser board, 2x E5-2667v4 CPUs, 256gb ram, GTX 1080ti for like $490. After using some other recycled parts and then selling some old stuff on Ebay, I ended up with a new computer for like 350 bucks... and for being essentially 10 year old equipment, it's shockingly performant.

You just never know what you'll find!

It takes time to research and decide what you actually want, then locate it at a good price, so you better be in love with the hunt hahaha.

[0] https://pcserverandparts.com

My favorite sources are garage sales, swap meets, thrift stores, and storefronts for recycling centers. I haven't had much luck with online sources.

Some bigger cities have electronics recyclers and upsellers. Look for them on Google Maps or such. Thrift stores will often have printers, routers, etc.

Best Buy actually has a good certified used section (open box and returns) that sells them for like 20% to 30% off.

eBay and Woot for off lease stuff. Micro form factor PCs and HP thin clients (t640, t530, t730)

If you want a raspberry pi type of project and don’t need gpio, the thin clients are awesome and usually smoke the Pi these days.

With the cost of m2 ssd coming down as it has, you can get a lot out of these little devices.

eBay is fairly cheap for commodity hardware. Especially the off-lease commercial desktop PCs.

Liquidation auction sites, estate sales, the like. Govdeals or equivalent.

A patient future of regular market monitoring accompanied by ready cash is the best preparation for scoring a lucky bargain.

Basically, you can trade time for money.

There is no website for easy arbitrage.

It also helps to be able to service/repair the tech you want...again trading time for money.

Paying market rate is fast and convenient.

Good luck.