The need for CP is definitely a factor and it's tough to balance things out. However, I do agree that it is better to sell off some cards and buy the ones you need. At least it will help but I haven't really felt the need to do anything like that yet. I like modern format more due to the lower CP requirements and I think I can just rent out the remaining CP I get when I actually have time.
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I was thinking I'd be able to sell off those junk cards by listing them for less in the market, I did, I also raised money in the process to actually buy some cheaper cards so I always see the need.