I have done both. I have purchases a plane from Lowes and tuned it up and I have purchased many old Stanley planes on eBay. You have to be careful buying on eBay. some are beyond repair. I have been able to get some great planes there though.
I do agree, however, that you are better off restoring an old plane than tuning up a bib box store plane. The amount of work is about the same but the old planes feel much better when in use.