Refurbished Macs from Apple Official - yes, realiable and they also give warranty
Second hand on Carousell - it all depends. You can check the pics, the physical item and most importantly, the reviews of the seller. If somone sells a brand new item at a very low price, with no reviews, stay away,.
In most of my trades (almost all my iPads have been bought used from carousell), many of the sellers just bought the items and never really used it / or rarely used it, and the conditions are very good. Especially ipads, they last a very long time and you never really need the latest and greatest, so its a great bargain.