CSWS- there is a fee/charge to load auctions and start them later- but you could use something like turbolister to get all your info in and then load when you're ready to go.
Check what price things have gone for that you want to sell- eg selling prices of similar spec/condition items. (using the advanced search on there). That should give you an idea of what is worth listing.
Try to build a little into your postage & packaging costs to 'pay' for your fees- not strictly allowed, but as long as you are not stiffing people on postage (which I never do) then it won't cause any problems.
The royal mail site has a form where you can check postage costs for weight/size of package so you can accurately quote P&P- ie not charging people £3 P&P to mail out a babygro but rather the 85p it costs plus 30p towards your fees, so more like £1.15...
If stuff is very heavy, but only goes for a low price, it may not be worth auctioning- however... you can specify collection only on auctions. (and make sure if you do that you have cash on collection only so that you know you have the money for it.... people could collect and then say they haven't received the item, and you would have no proof for paypal, and they would find in buyer's favour for example.)
I think Frizbe sells on there too, so she probably has lots of tips.
You'll get a better price for branded goods- ie 3 JL sleepsuits as opposed to 3 tesco ones, but that goes without saying.
What are you selling [nosy emoticon] ? some things are not worth listing.
Cheapest listing fee is for starting at 99p.... but you have to be prepared for an item having only 1 bid- start price really should be the absolute lowest you would accept for an item... as lots of new sellers find to their cost
I have made reasonable sales in the past... but there are better times of year to sell certain items IYSWIM- eg selling summer clothes in winter is stupid, as bids will be very low... however selling them in August is too late, etc.
HTH