I sell almost all clothing. I actually have a storage unit full to sell. I am a reseller. I shop rummage sales, garage sales, and thrift store sales for deals. I LOVE bag sales at rummage sales.
I usually have 40-50 items listed at all times. I sell an average of 75% the first time they are listed. After 2 listings it goes back in a bin to either be regrouped, or sometimes it goes back to donation.
I keep track of everything I make and all my costs. I don't usually sell in lots of more than 2-3 items. I found that grouping a skirt or pair of pants w/ 2 tops is usually more profitable. People seem to like to buy outfits. I found you can sell almost any brand- but you have to be ready to accept $.99 for some items and that might mean in the end you only make $.25 depending on how much you "pad" shipping. Which is a whole other topic.
It does take a lot of time though to make any serious money of clothing. Unless you have high end brand new w/ tags items. If you have the time though, it can start to add up fast.