One small detail the article forgets to mention, you will still need at least a 256GB card and pretty soon a 512Gb one to run the full node, the package doesn't come with one and the cheapest is around 30-40$.
Funny how the site they mention
http://www.raspberrypifullnode.com/ still talks about 64 gb

TBH I'd use my HDD docks and just place my unused HDD in there if I were to run a full node. Pretty sure people with the knowledge on what to do with Raspberry Pi and how it works would have had some extra PC components lying around and I'm one of them (lol). Anyhow, knowing that $/GB has been pretty much going down year by year, it's easy to find the right storage device if one is to run a node. Everything is coming cheaper for the PC industry these days, well at least the outdated (?) components.
Used HDD? I wouldn't use it, especially to run full node & leave it 24/7, who knows the remaining life of the HDD.
True, I wouldn't buy one but if I have a spare lying around, that would do. Even if you can utilize the help of HD Sentinel and other similar software, I'm still not comfortable buying used HDDs for important uses, at all.