bigdata
I wouldn't say that it should be avoided. It just comes with some additional considerations.
First, and probably most importantly, make sure it is an OEM unlockable. Either do this by buying from a reputable seller with an easy return policy and accurate product descriptions, or by requesting them to check and show you that OEM unlocking is available in the Developer Options settings.
Past that, maybe look for sellers that include a warranty. For example, Amazon renewed devices often come with a 90-day warranty; Ebay Refurbished comes with a full year warranty.