I’m going to give you tips on how to do your listings for your item descriptions (ie. writing them up, on each site). Some sites are a little more technical and precise, while others not so much.
For ebay sellers, you really want to have every possible detail mentioned; for example, color, material, size, condition, if you are the original owner/item history- how you acquired it. (Black Lux Small Juniors Cotton Blend Bubble Skirt, excellent used condition- worn once- purchased at Urban Outfitters.) If listing authentic designer bags, you might want to include the creed number/registration number (Black Leather Small Coach Hobo Bag- good used condition- purchased at Buffalo Exchange- creed KHI- 954). This is not required, but it is helpful in building buyer confidence. Sometimes sellers post links on how to spot the fakes, as well. You also want to have detailed bidding terms (ie. seller conditions- paypal preference, how often you ship during the week, if returns are accepted, which shipper you use, etc.). A listing with pictures gets more interest.
For listia sellers, you want to have every possible detail mentioned; just as it is for ebay- color, material, size, condition, if you are the original owner/item history- how you acquired it. When listing authentic designer bags, you will need to post creed pictures, especially when it comes to Coach products. For Dooney & Bourke, you will need to flip the red, white, and blue label over, and snap a picture of the registration number. In addition, mention the appropriate size, the approximate measurements- Length/Height/Base width, strap drop, whether anything is adjustable/number of holes in the buckles, pockets- exterior and interior, type of closure- zipper, snap, open, type of hardware- brass, nickel, if anything was modified, if there are any flaws, like stains, marks, worn out piping, etc. Also, the moderators will ask you for additional pictures, if this is not provided. If you do not respond, your listing could be flagged and pulled off the site. In fact, there is a sort of encouragement on the site, with offering badges for flagging people’s items. If you have detailed item descriptions, you will have less irrate comments. Sometimes the people on this site can be extremely rude, so expect comments, if there is any inkling that an authentic designer bag is a fake. (Yes, I am speaking from experience.) Out of all the sites mentioned here, there has been intense scrutiny, as there is a strict policy on listing fake designer items (while designer inspired items are alright). I notice that there are less bids on items without pictures.
For swapstyle users, you can list a $ value, as the site allows selling in addition to swapping (but some swappers take advantage of this, by using retail values for used items, just an FYI). You can use the guidelines already mentioned, to avoid any confusion. Certain details are required when listing items. I like including the size in the title, so it saves time.
For rehashclothes users, selling is not allowed on the site. You can also use the same guidelines already mentioned. Certain details are required when listing items (title, trading with, condition & description) but the other fields are optional (size, category, looking for, pictures, tags). I like including the size in the title, which saves time.