Clothes can sell very well. This is what has worked for me:
.Higher end brands like Boden, Joules and Next sell well individually whereas supermarket type brands sell better as a bundle.
.Try and go with the season so spring / summer items will sell well now.
.Try and make bundles of outfits or items that coordinate rather than a bundle of random items.
.Photos are your friend! Get a nice blanket or duvet cover to lay your clothes on (as a background) and try to make sure nothing but that background is in the photo. Wash and iron the clothes so they look really nice in the photos and take your photos in bright, natural light so the potential buyers can see exactly what they are getting.
.Be detailed in your listing. If you just put 'girls clothes age 2-3' the buyer does not know what they are getting but if you list items individually e.g:
Next Cord Skirt age 2-3
Next long sleeve top age 2-3 (matches skirt)
. Be honest. If something has only been worn twice, say so, equally, if an item is 'well-loved' and is only suitable for play-wear, say so too.
Hope that helps.
I've never sold books, so can't help you with that, sorry