Many sales will have coat hangers you can have if you ask in advance, or try freecycle. Most sales now sale all clothes have to be on hangers, but check the guide.
Agree with moomaa (another long term NNS helper here!) I would say that clothes need to be very good condition, good value and decent brands - supermarket clothes won't sell unless you do a good bundle and price very low.
Putting together outfits (plain top under a dress; top + trews) also helps things sell.
Toys, jigsaws, books and equipment are easier sells, but for bigger items, need to be priced slightly lower than you might sell on ebay (people are in 'bargain' mindset usually. Again bundling things (eg set of baby rattles) can work well.
Make sure that everything is clearly labelled/fix bits firmly on -there is a scrum, and it's easy for price tickets come off/things get pulled apart then they'll be taken out of the sale.
Spending a bit of time making things look nice ito presentation (clean, in decent box, with clear attractive label) makes a big difference, imo)
HTH!