This won't really be a guide as such, moreso just a couple of quick tips in regards to posting 'Want it Now' listings.
I sell more than I buy on ebay, and I quite often check the Want it Now listings to see if I am in posession of any items that people are after. I have sold about twenty items thus far to Want it Now buyers, but one thing I've noticed among most listings, is that people don't name a price that they want to pay.
Initially if I saw a listing where someone was requesting an item which I had and was willing to part with, I'd send through a message saying that I had the item, and asking them how much they were reasonably willing to pay. If they messaged me back and named a price that was fair to me, then I'd list the item for them to buy.
The last half a dozen or so times though, I simply haven't recieved any messages back, so that's discouraged me quite a lot.
I now very rarely respond to any 'Want it now' listings, which in theory sounds stupid, because honestly it only takes me a couple of minutes to send through a message right? It's just that I no longer feel encouraged to.
I imagine that many sellers feel the exact same way as I do, meaning that potential buyers simply aren't being offered certain products they want, even if sellers may be in posession of such items.
So just a couple of quick tips if you are listing in the 'Want it now' section of ebay, and want to attract responses from people who may have the item you're desperately seeking -
First of all be specific about what you want. Don't simply say "Wanted, black dress". Describe the item that you want. Give a little detail so that potential sellers will have a fair idea of what you are seeking.
Secondly, it never hurts to list what you are wanting to pay. I gather that a lot of buyers don't list prices, just incase there's a seller out there willing to part with the item for less that what they are initially willing to pay, which is awesome for you. But in all honesty, for the chances you might have of saving some money, you're running a far greater risk of not getting responses at all.