The above suggestions are all excellent!
I can't stress enough the importance of providing as many accurate measurements for each garment as possible, in inches and cm - bust, waist and hips measured flat across, length collar to hem, waist to hem and inside and outside leg where appropriate. Sleeve length shoulder to cuff is also important for jackets, and mens shirts.
Sizing varies enormously even within a particular brand and the only accurate way for a purchaser to know whether any particular item is likely to fit is to provide as much detail as possible. This is particularly true of vintage clothing - it drives me batshit crazy when clothes are listed as for example a "size 14" with no measurements given. A 1970s size 14 was 36"-26"-38" which is a modern size 10!
I include a key measurement in the listing title when clothes are labelled S,M,L, e.g. a man's T-shirt would say 'Size M, chest to 40"' in the title.
If there is a mismatch between label and measurement, e.g. a garment with a size 16 label and a bust measurement of only 39", I would describe the item as as "size 14" in the listing title, and in the description I would explain garment is 'labelled 16 but a small fit, bust up to 39".' People usually use their dress size as a search term, and might miss out on your item otherwise.
I also offer free 2nd class postage and I make a point of emphasising that I always get proof of posting, I have NEVER had an item go missing or 'missing'
(touch wood!) This means my start prices are slightly higher than comparable items, but I don't get quibbles about postage costs or delivery times.
Try 'cross-selling' . If you are selling a large amount of stuff, put a note on each listing to say:
'Please check out my other items - am selling lots of lovely clothes and shoes in sizes 10-12 suitable for someone about 5'6", shoe size 6, Also boys clothing age 6-8. Brands include ABC and XYZ.'
This works particularly well if you are listing houshold items for collection only. If a buyer is going to drive to your house collect some curtains, they might well take the mirror or the cushions you want rid of for a few quid while they have transport.