I have done a lot of wheeling and dealing on eBay (partly selling old miniatures for new armies and partly for fun). Overall I have made 44%.
Here are my tips for buying Warhammer miniatures:-
1) Search for "Warhammer" and "Miniatures" and "Citadel", not just for the category you are interested in.
Some sales are made by parents, people well out of date with new miniatures and toy/game shops all of which do not always list their sales correctly.
They usually know that the metal figures are Warhammer or from Games Workshop but they do not always know the appropriate names.
Search for :-
"Games Workshop"
"Warhammer"
"Metal figures"
"Warhammer miniatures"
"Citadel miniatures"
"Citadel figures"
"Warhammer"
"Metal figures"
"Warhammer miniatures"
"Citadel miniatures"
"Citadel figures"
and all of the above spelt incorrectly, especially "War hammer" or "Warhamer"
You will find figures for sale which other people would like to find but are searching much more specifically for.
Also when searching for specific races, do not just use the GW standard naming conventions...
2) ...Use all race names
Do not look only for Dwarfs but look for "Dwarves" aswell. Likewise don't look for "Elves" look also for "Elfs".
Are you beginning to get the picture ?
Lots of lots are listed wrongly and there are lots of cheap figures out there if you're patient and smart.
Example: I bought 2 units of Dwarfs off a person who advertised them as, wait for it, "small elfs" (!). I probably got them for about 1/3 of the price I would have paid if they had been listed properly.
3) Be smart about when you buy
Everyone buys and sells on a Sunday afternoon or a Saturday evening, that is, if they're smart and have the time.
Look for listings that end midweek, there is a LOT less competition. Most sales end on a Sunday and people will be sitting by their keyboards "sniping" (ie going for last second bids to get a bargain). Also, there will be that person who is absolutely determined to get that lot, they will make sure they get it, money is no obstacle for them so make sure you're not in competition with them !
Example: I wasted a lot of money by always buying on Sundays instead of being patient. Look for week day end date instead and lots ending in a single day rather than those advertised for 10 days
Example 2: I bought 5 Vintage OOP Metal Ogres for £6 by purchaing off someone who had listed the sale to end midweek in the morning !
4) Know your limits
Decide before hand the most you would pay for a lot if you saw it in a shop and could buy it there and then. Do not exceed that limit !!! For all you know the actual seller has opened another account and is bidding against you on his/her own lot.
Example: We have all been guilty of this one, we decide we want it and cannot stop. It's a double edged sword, if you sell figures you will benefit more if you can keep your cool when you're buying.