Refurbished is the safer choice in almost every case, and that’s especially true for tech accessories. The value of refurbished lies in the process behind it. A reputable refurbisher will have tested battery health, checked for faults, cleaned the item properly and assigned an honest condition grade. When you buy cheap tech accessories from an untested private listing, you have no way of knowing what condition the battery is actually in or whether the item has any hidden faults.
The trade-off is price. A private second-hand listing will often be cheaper than the same item refurbished from a retailer. Whether that saving is worth it depends on how much risk you are comfortable with. For low-cost items, it may be fine. For AirPods, smartwatches or an Apple Pencil, where the battery and internal components matter significantly to the experience, buying refurbished from a tested and graded source is the better long-term decision.