Well it also depends who you want to market your stock to? To other hobbyist or to wholesale to stores? Also your budget & tank(s).
To the hobbyist, the rarer the better & get more bang for you buck. As in the Sting rays etc.. Most cichlids are pretty easy & can have more then you can sell at times? Plus when you sell to the average person they might only buy one or a couple fish so you should ask a little more $ because your time, energy, & cost to maintain your tanks in turn cost you.
To the stores the more the variety the better? Typically stores will buy much more and spend a couple hundred bucks or a lot more at a time in which you move more stock quicker? There is supply & demand. Whats hot & whats not?
Quality fishes is # 1 to who ever you sell to because if they are not good, your reputation will be judged by so. Be consistent on quality, size & price.
Angel fish, rams, guppys, cory cats, bushynose plecos, are some of your bred & butter stock of most if not all stores and need these fishes everyday.
Rift lake cichlids especially Malawian as in Electric Yellows, & electric blues & the peacock group are pretty easy to breed, but you would need to educate some of the retailers that you may want to sell to so they know what is needed to know & how to sell them. The more they know & sell, the more they are apt to buy from you on a regular bases.
For us we specialize in cichlids and breed them in our own hatchery so we do not buy to much from breeders but on occasion. Usually I buy the bred & butter stuff from local when I need it?
Back to supply & demand. For us the summer is slow & we have a hard time moving fish so you may have the same slow times in your area & thats when you par down & grow out stuff for the future busy season.
You could also sell to the wholesalers/distributors but you will take much less per fish but can move a lot more stock at once.
Selling wholesale also you may have less headaches because you sell to a couple places or even one instead selling a fish here and couple there.
I know since I have been a hobbyist by blood for about 30 years, in the industry for about 24+ years, so I have seen it from most angles? Sometimes I miss doing it as a Hobby on weekends. Remember the more tanks, more fish, more cost, more work, more chances of losses.
my 2 cents.
me Dave