It really depends on the price.
Used CPUs, generally are ok - they don't tend to "fail" like other components do...... although with unlocked skus, it's impossible to know how hard it was run, what voltage it was run at etc etc.
Ask lots of questions, make sure the answers make sense.
Ask if it was overclocked, if so; what voltage? What speeds? What were temperatures like? What motherboard was used?
Between a 4690K and a Ryzen3 1200/1300X, the 4690K is the stronger performer.....
Considering you'd need a new Motherboard & DDR4 (ideally higher speed), you should be able to find a 4690K for much less.