Unless low quality items have some known quest use or are coveted by other players, you should try to sell it as soon as possible to create bag space. Always (or almost always, see above exceptions) keep things like cloth, leather, herbs, or large stacks of white/gray items over other loot when you have to decide what to keep when your bags get full. It might be worth your while to invest in larger bags (10-20 slot), especially if you know a tailor.
Most of the players make the mistake of only listing 30-50 transmog items on the auction house and then hoping to get half of them sold until the next day. Me and a lot of other gold farmers and transmog sellers do have 3.000-10.000 items on the auction house across various realms. It does not mean you have to do it that way – but if you have 1000 items listed, chances are higher you can sell up to 10 per day. With only 100 items listed, you have to be lucky to sell one. Of course, the sells are not guaranteed if you post more items. But i’m sure you have a lot more fun if you open your mailbox and 10-20 items are sold