It is far easier to sell a good bird than it is to buy one. Having bred a reasonable number of youngsters, you can cull quite ruthlessly once they have moulted out. If you bought cheaper birds from a good stud, a number of their youngsters should be much better than their parents, and it is these birds that you must work with, together with the adults that bred the better quality youngsters. Depending on numbers bred and the number of birds you want to retain, you should dispose of those that do not measure up. It is no use keeping inferior birds bred from inferior birds if you want to progress.
Bloodlines are fine but even the best ones can be quickly bred out.
Poor birds bred from good birds can produce good birds but rubbish bred from rubbish is more than likely to breed more rubbish. A word of warning, whenever you sort out birds for disposal. Place all the birds that are to go in a separate cage or flight and leave them there for a day or two, casting your eyes on them whenever you pass them. The same can apply at a show and sometimes accounts for inconsistent judging. Once you have sorted out your birds, you must go over them and see if there is a feature that is obviously lacking.