Some people think nothing of buying clothes that other people have worn while others can find it a bit wierd, but whether you are on a budget or that must have item is sold out in your size, second-hand is not necessarily second-best...
Personally, I'm partial to Karen Millen designs but find that the shops hardly ever stock my size in the designs I like best and when they do, well... I choke a little at the price. I know there are a lot of women who will buy clothes and then do not want to be seen in the same outfit twice and to my mind, this is no different to buying something from a shop that a potential buyer tried on and then decided not to buy... except a LOT cheaper!
Then of course there are the brand new, with tags (BNWT). We've all been there, bought something that doesn't fit but we've lost the receipt and can't take it back, or it was an unwanted gift. For these items, buying on eBay is a no-brainer - it's just like it would be buying from a shop but cheaper and it gets delivered!
Of course, you have to be careful with clothes described as used. Check the listing carefully for any mention of damage: missing sequins, pulled threads, stains, ironing damage etc. If you have pet allergies then you may want to ensure that the garment comes from a pet free home, don't forget you can always ask as not every seller will mention this!
Another thing to be careful of is size. Sizing varies from store to store and a 14 in Karen Millen may not be the same as in Per Una or Monsoon. Many sellers may not accept returns just because of the wrong size. Again, if the listing does not make it very clear, don't be afraid to ask.
It's not just ladies clothes that can be treated this way. We all know how quickly children grow - do you think they care whether the trousers they are getting covered in mud are new or used? Save yourself the time and hassle of dragging your child(ren) around the shops and get their clothes quickly and cheaply. The same can be said of Gent's clothes but that's kind of outside of my personal expertise...
By buying once worn clothes you can save yourself a small fortune and look great!