Do your research about the delivery service you'll use. Hermes is priced very competitively although it messes up from time to time (my experience is about 2%) and when it does they do have a tendency to lie with their excuses. Even with this down side I do use them because it really saves money.
If you have a personal account you'll get 20 free listings per month. You also get the free option to schedule their starting times. Different people favour different ending days but one important thing is that if you're going to list as auctions and not as Buy-It-Nows you should aim for your auction to end at a convenient time of the day, say after 8pm.
Unless you operate in a dodgy evasive way, ebay take a 10% cut. Unless you're having people collect from you and giving you cash, Paypal take 3.4% +20p (if I've remembered correctly), so that's nearly 15%. Since you can be spending £3 at least on delivery, then there's packaging materials (if used), you can see that if you sell an item for £10 including delivery, you might be getting around £5 back.
You might want to use settings to restrict to UK sales only.
Second hand clothes seem to attract a large number of chancers so have good clear descriptions detailing faults/wear and add lots of photos.
I sold my ten year old bike for £80 and it started at 99p. I'd say always start at 99p especially for used clothing unless it is an item you really want to keep hold of unless it makes the minimum. Always weigh the items and packaging so that you can put a realistic postage price.
If you sell at 99p after fees then you will make zero profit - the only reason to start at that price was when those listings were free. Now you get free listings at any price why still start at 99p - is utterly pointless and a complete waste of your time. Plus if you only have one bidder that is all you will achieve.
If auction do end in the evening, or do a cheeky buy it now price but add best offer to it (buyer will think they have a good offer when you accept their lower price). And free p&p is good too - so up your start price by £3 for Hermes (if under 1kg) and then you guarantee 5 stars for postage too.