Like many of the other members have posted I also don't feel that bowls are the best environments for bettas. Sure, they can live in one if you perform constant water changes and a heater, but they will not thrive. If I'm going to keep a betta I want it to thrive not merely survive.
I have a 4 gallon drum bowl that my betta loved until I upgraded him to a divided 10 gallon. It wasn't hard to heat or anything, and it looked really nice.
That's the exception to the rule. Most bowls are 1 gallon or less, so you can't fit a filter, its too small to cycle, most heaters won't fit, and most heaters will overheat the bowl and kill the betta. They don't abide by the two musts for keeping bettas 1) space 2) heat. You can get by without a filter, live plants, even substrate, but they need adequate space and heating.
In my opinion, bowls are fine for bettas. UNLESS you have them in a half gallon bowl without a filter or heater. Like JellOh said, if the bowl is big enough and it has the right components needed to make up a good tank, such as cycling, enough room, heat, filtration, ect., I think a betta would be fine. But, if you put a 4 gallon bowl with heat, plants, filtration, cycling, and substrate against a 4 gallon tank with the same attributes, the tank would probably win in the betta's opinion.
I suppose a bowl could be okay IF it was at least 2.5 gallons, and had a heater, and the owner maintained appropriate water changes.
BUT I've never really seen a bowl that size, and most stores lack on educating their customers about proper water changes.
I like tanks because they are easy to decorate, have lots of swimming space, can by cycled, and it's easy to watch my fish swim (bowls distort the view and that annoys me!)
I'm a breeder as well, and if you have ever seen a betta in a tank you will understand that bowls are not the best. They are notoriously hard to heat and often they do not give enough room. Lots of water changes are also needed, like every second day. And please do not use this as an advertisement opportunity. The "classified" section is made for that :) Also, I'm concerned with how you keep them and the bowls you sell.
My 1st betta (Sushi) was in a 2g unfiltered bowl with a heater and live plants. I know he was a little older when I got him b/c he was already grey around the gills. I changed his water 100% every Monday. Had him for 9mo. Without any signs of illness I lost him about a month ago. He was fine when I went to bed and gone in the morning. Still dont know what caused it. He was never a very active fish, he spent most of his time sleeping in his plants. He would flare like crazy when I came near his bowl but then he would settle down and go back to sleep.
Needless to say I decided to get a better set-up for my next betta. My new guy (Chum) is in a 5g filtered tank with no live plants this time. He is CRAZY active, I hardly ever see him sit still. I will admit that he is younger than Sushi so that may be part of it but alot of it is he just has more room and hence more stuff to explore. I can say that I've been converted. You CAN make it work in smaller bowls/tanks but Im happy with my 5g and I think Chum is too. Wish Sushi would have been able to enjoy it. maybe he would have lived longer but who knows....just my 2 cents worth
While we are talking about tank size I feel I must point out that guppies need a 10gallon tank because they are so active.
No kidding!! I feel like my guppy's have some constant source of crazy juice! they are some swimming fools all day long! And love to ride the filter flow roller coaster haha
However.. back to bowls. I actually created a bowl from a ginormous candle votive. Was a really pretty QT bowl for all our fish(except the Guppy's) for their first little while before entering their tanks. It took a little over two gallons to fill and has become a backup hospital tank(in case I need more then 1 at a time.)