I agree with the advice above- start with a low listing on ebay, you are sometimes (not always) amazed by how high things go for. My experience is that specific brands go better than anything non-specific- people tend to search by brand name and size e.g. Marks and Spencer size 22 or per una size 18.
With the car boot- fix in your mind a price you would be happy with, then add a little bit and allow yourself to be haggled down. Suits, coats, 'party tops' and pretty shoes seem to get the best prices- workaday shirts, t-shirts and trousers rarely sell , similarly named jeans often go well, non-brands from supermarkets are not usually worth the petrol it costs to take them. But that's just my experience!