With clothes, I rarely buy pre-owned items. When I do, it will generally be something that is worn as outerwear and only from a seller with high ratings. Based on the sheer amount of used Lululemon clothing on eBay, I assume there must be a high demand for it but there is no way would I buy pre-owned exercise clothes. I KNOW what mine are like used, so I could not fathom buying someone else's grungy sweats.
Along those lines, you really don't know what state an item is until you see it with your own eyes, regardless of what condition the seller says its in. Their idea of "Mint" or "Like New" may really vary from yours. Which means that you are taking a chance, and hopefully that is reflected in the price. If you have experience with a seller, or if they have enough photos to prove something is in decent shape, then go for it. But if those pictures are too dark or out of focus, beware. Photos, more often than not, are my biggest decision maker.
I would like to add that some rare items, however, are even better pre-owned. My biggest addiction on eBay is hunting down Tiffany & Co. jewellery. Their silver jewellery only grows more beautiful with patina that develops over time. I will discuss authenticity in another post but for this topic I have to say that I almost prefer buying this second hand over brand new. First off because you won't find new authentic Tiffany jewellery on eBay and also because I see Tiffany's as a collector's item with increased value over time. Again, reputation and photos play a big part in my decision to bid.
All that to say, that it really depends on what you are looking for when deciding to go New or Pre-Owned on eBay. I try to avoid disappointment and opt for new clothing/shoes but I will accept used when it comes to jewellery or purses IF I can see many photos to decide on its state for myself.