I bought a Bonsai last week, pretty sure that its a Chinese Elm. But the seller told me to never repot the bonsai. And I according to experts, bonsais should be repotted. Well, the roots are quite abnormal! What must I do? Please Help me! Must I repot it? I live in a tropical country, so its always hot! So when to prune & fertilise? thnak you

Welcome. Yes of course they must be re potted, anyone who say otherwise knows nothing about bonsai. It is Chinese Elm, and it look very roobound from the look of the nebari. From what I can see it nees to be re potted sooner not later and if your conditions are such that you do not get a colder season the it can be done almost any time. A slightly larger pot would help and it would allow the roots to be spread out. It does look healthy so that is a good thing.
What country are you in?

I'm from Mauritius. Thank you very much.

The only thing that makes me more angry than people that sell bad trees, is people that sell bad trees, know nothing about bonsai, and give ridiculous advise.

Listen to m5eaygeoff, it is the correct advise.