should I fix bike or sell? braking problems?

I would turn it in for a rebuilt. If you have the parts you can rebuild but not the machine shop standards. Another good choice is to get a single used caliper. For best practice, both calipers rebuilt or new. Also, watch out for collapsed hoses which can look like stuck piston.

Push the brake piston back into the caliper and try using it again. It likely went crooked and got stuck. If that doesn't remedy the situation, the caliper may need replacing. Changing out the caliper and bleeding the brake line might take an hour.

I don't know what a shop would charge since there are so many different variables. But to answer your main question, if you enjoy your motorcycle and the thing is in good condition for appearance, frame, and looks, it makes far more sense to just get it fixed. Brakes are a wear item that have to get replaced, and calipers do wear out and seize up in time and have to get replaced. The next one will last for many years. If you're mechanically inclined and have a service manual it's a job you could do yourself. But if not, take it to a professional and have it done right. It may cost you a bit of money but it would be far less than buying another motorcycle.

And if you don't enjoy it and/or rarely use it then yeah, sell it, but you'll get a lot more money for it if it's fully working and has good brakes.

Fix but burning is more favorable choice.