Does anybody know whether it is trustable
They seem to have partnered with Rome2Rio which is a reputable and frequently used website.
For test purposes, I put it in a trip that I just travelled yesterday and it found good but not the best options.
and we can easily buy tickets?
Probably yes, but I personally always buy the ticket directly from the provider. My recommendation is to use this website to find the best (or good) option and then book and research the provider directly.
Is it risky to buy ticket in person while going to destination?
It's not risky as in "dangerous". There are downsides: buying in person you typically pay more then buying online or upfront. The bus can be full and there may be no seats left. Payment can be tricky: You may not have the right denomination/change for counter or the ticket machine. Lines at a counter can be long and so you miss your bus.
I mean in this season could we find an empty bus if we wait to buy in person/?
Unlikely. If you want to get a feel how empty a bus is you can try to track seat availability through online booking tools (at least for some busses). However, things change quickly. Your best shot is to book early and reserve the seat you want.