I have run a few of these for the NCT. You'll generally receive 75% of the sale price - 25% is a donation to the charity/covers costs etc. If you have the right things to sell and price them right you can do really well - we regularly had people make £300+.
Toys, books, videos go really well particularly at this time of year (in fact, we did a toy only one in the run up to Christmas a couple of times). Goes without saying they need to be decent quality, and don't overprice things (about 1/3 of 'new' price is about right, unless it's brand new or something that is really in demand). I wouldn't waste your time sending soft toys tbh, they don't sell well.
With clothes, keep them seasonal (coats and wellies etc will sell well at this time of year) and don't expect too much. Outfits will sell OK as will anything that is from a recognisable brand name or designer. But you are unlikely to shift vests/sleepsuits/really ordinary stuff (it's so cheap new from tesco etc) - although you could try doing a 'bundle' for a one off price.
Equipment - you'll have to check with the organisers, but we didn't take any electrical equipment or car seats and in some venues we didn't have room for big things like pushchairs although we did have a 'sellers board' once where people paid to advertise bigger items.
You do need to put a bit of time into selling, if you treat it like a jumble sale (and lots of people do) you will get nowt, ime. And make sure you follow the instructions about labelling really carefully or your stuff might not go out on display.
Also, it can work to volunteer to help on the day as you can make sure your stuff is well displayed (there have to be perks for the helpers... ).
HTH