Thing is, buying isnt the issue, its selling, just sold a motherboard to a an absolute idiot who is now trying to return it due to him probably F'ing it on the way in or something, he claims the RAM ports are dead but worked 100% putting it in!
ANYWAYS, second hand is always good, and generally you can get some very very good deals... Just have a check if on shmebay at their recently sold items to check for mining cards as some people absolutely wrecked them! Basically look out for them selling anything suspicious such as a stupidly low end CPU with a motherboard with loads of ports, and tons of mostly R9 290's (or just AMD cards)!
Just be careful, check their feedback etc and ebay always rule in the buyers favour, and thats why I hate selling on their, a lot of scams happen, but also a lot of genuine people, take your pick and investigate before buying to avoid hassle!
To summarize, yeah you can get awesome deals on 2nd hand places, just be wise when buying!