IMO-it depends on your goals and as Myates pointed out, understanding that you never know what you will get.
A Betta either meets standards or it doesn't and it may take several generations to reach your goal. With that said, IMO that is half the fun-especially when starting out with mystery genetics.
When starting out with mystery genetics-IMO, you should plan to cull hard-really hard-down to about 20 fry or less. This is a good manageable number to work with and less to find homes for. Every area can vary regarding the demand for the unique Betta-Some Ma&Pa
pet shops will take them to either sell or to use for feeders-you may or may not get any real money-some will give you store credit-but that won't be much if anything.
But as far as using pet shop Betta for breeding-I personally don't have a problem with it and the age factor-IMO/E I have never had any issue with spawning my older breeders.