Pick brands and designers where the clothes are well made to start with and where you know how the sizing works.If you are buying something by a named designer, be aware of any catalogues they may also work with, get familiar with how the labels look so you can spot them in pictures.
Look carefully at the pictures. You will start to recognise which designs/items are current/last season and which ones have been tucked in the back of a wardrobe for a couple of decades.
Think about what the seller isn't telling you. I avoid items where the seller hasn't bothered to add any detailed description.
Remember to factor in P&P and don't be tempted to bid more than you are comfy with. There will be another similar (if not identical) item along shortly.
You can set up searches and alerts on ebay, so ebay will let you know when items you are interested in get listed. Bide your time and buy only items where you feel comfortable with the description and the seller. Be picky.
Be aware that clothes often arrive smelling strongly of other people's washing powder and conditioner. This is a good sign (because they are clean) but I personally find some of the scents revolting, so I do a quick try on, check out the item and then get it in the laundry asap.
If you don't like an item you've bought, you can always bung it back on ebay and recoup your costs.