I buy quite a bit from ebay, both new and second hand. Private sellers and shops, it's all the same to me. I've largely had good experiences and only the very occasional bad one. However, I'm pretty careful about what I buy from where and how much I pay. I suspect I've had marginally more negative experiences buying from established UK fishing retailers (direct and via ebay) than anything else. More than the odd one is prone to selling things they don't actually have in stock. I've also had my suspicions about a few auctioning items, not getting the price they expected, then claiming the item is out of stock. You'll usually see evidence of a slightly dubious seller in the feedback, even if a superficial look suggests that their feedback is pretty good. However, always bear in mind that there are plenty of idiot buyers giving unwarranted negative feedback. There also may be instances of understandably disgruntled buyers never getting to post warranted negative feedback.
Unfortunately, an awful lot of buyers seem to lack patience. They often experience problems with couriers and complain about the seller. I don't generally expect immediate dispatch and next day delivery, especially from private sellers. I anticipate that most will have proper jobs and won't necessarily dispatch as quickly as they possibly could. The only real complaint with that is that they could often communicate better or be a bit less naive with the system generated expected delivery times. If you want an item next day, or the day after, chances are you are going to have to buy new from the handful of mail order outlets that are well known for such speedy service. Bobco and Tacklebox spring to mind as being amongst those that consistently get this right. However, even the outlets with good reputations can have minor difficulties on occasion.
I also take the use of terms such as mint, unused, new/other etc with a bucketful of salt, especially from private sellers and dealers. With fishing kit, it really pays to look carefully at the pictures. Claims of kit being new other/unused are often easily discounted. In so many cases sellers are too lazy to clean duplon/cork handles. They leave plastic on old rods thinking that it proves no/little use when there are obvious signs of it being a lie. One of my biggest bugbears is people propping rods up against garden sheds and fences with butts and female sections on bricks, concrete, paving, gravel, grass, or muck. If that's how they treat a rod they want to sell, how badly did they treat it in use?