The first thing would be how you're going to sell them... locally or through shipping.
If you're going to ship, then Aquabid might give you an idea. If locally then as above it really depends where you are, and if you're going to sell as unsexed juvies, adults, proven pairs, show fish, etc.
there are lots and lots of variables, if you're going for the higher end or showier fish, then what's currently 'trendy' will be a big issue as well.
Probably your best bet will be to look at the market where you'd plan to sell.