Dig through earlier posts on this site as there have been some good ideas mentioned on here before.
My two suggestions are:
1. Amazon 2nd hand market. Pick the CD's you want (compilations best option) then click on the button that takes you to the Amazon market/available from other sellers. Many will be available for £0.01 plus the compulsory £1.21 carriage. Zoverstocks is the biggest and often the cheapest. I'm guessing he makes his money on the postage. £0.20/0.30 profit per CD doesnt sound much but if you sell them by the bucketload then it's worthwhile. I've bought loads this way and they are all very good. Damaged cases, lost labels and minor scratches don't affect playback.
2. Ebay bulk sales. There's always someone selling their entire CD collection having copied it onto iTunes. These are not always listed in the CD section. You have to hunt around for the wholesale / bulk sale sections. These guys can do it because they're not then using them for public performances. The market rate for bulk buys works out at about £0.50 per CD including carriage. So for example, this guy
http://cgi.ebay.co.uk/375-used-cds-mixture...=item3ca87970afis selling 375 albums or thereabouts so should expect £190 or so for the lot.
Beware of dealers that sell bulk but don't mention what the albums are. They're usually rubbish with a sprinkling of good ones. You can spot these because they have hundreds of feedback points and always other CD job lots for sale. If you can find a seller with a low score and only one sale with CD's on it then it's definitely worth a punt.
The trick then is to buy it, keep the good ones and relist the rest (often 50%+++) as someone who is clearing out his own CD collection (but I didnt tell you that one)
I have found that there is a saturation point on just how many CD's/downloads you need to buy. When you're returning into the game (like I have done) it's easy to panic buy anything and everything. 95% of my 10,000 tracks will probably never get an airing.
There is a hardcore list of tracks that every DJ must have (actually that might be a good new topic in the playlist section) and if that's enough to cover say three hours of hard dancing then you're almost there (background stuff isnt as important as long as it's pleasing to the ear and relevant). If say you get an average of 3 minutes per track (bearing in mind fade ins/clips etc) that's only 60 singles.
I'm betting many DJ's have their core playlist that they use (like me) at most gigs because they know it works and just add/remove as the trends change.
Sorry, went on a bit there, but I've had to learn this lot the hard way and more than happy to share my experience with fellow MDJ's
Let me know if this is of use.
Teez