I have done and still do buy collections that I can add some to my own and then be able to sell off some of the rest. The key is to know what you need and want, more than the price. If you collect Mexico, then look at Mexico collections until you find one that can enhance your collection and then look at the price/bid. If it is too much, or there would be too much to sell off, then pass. Patience will reward you if you just wait. To give an example ( as I don't know how much a person can afford to spend) look at this
ebay collection
http://cgi.ebay.com/British-Samoa-S...NH_W0QQitemZ180242305338 There are several early ones I need and also some that would be duplicates. I only collect these to 1940, so all of the latter stuff, much mint is topical, sports, birds, sea shells, subjects a lot of people collect, and Samoa,Solomon Is. is more intriguing than Romania or Hungary of the same stuff. So to me this would be a good lot, but I have set a bid in my mind that if it goes over, so be it. don't chase once you have set a bid in mind.
But this is how I approach obtaining collections I will cherrypick. I love it when I get a collection , even if very small, that I only have a few stamps I don't need. But put "needs" before quantity IMO.
Jim