Based on the halacha that the overriding principle in commerce is: hakol k’minhag hamedinah, shouldn’t Dayanim have to learn basic laws of commerce that apply to their local municipal, or do they just send these cases straight to non Jewish courts for them to deal with it?
And if you say that obviously they don’t need to know the local law to judge the litigants by it, but do they at least need to know local law in order to know that it’s not a bais din case and needs a local court , and therefore send/allow the litigants to dispute by the local court?
Local law often plays a role in deciding the halacha, and as such the Dayanim should certainly be familiar with it or at least have the tools at their disposal to learn about specific laws. However this is no reason to have the case judged by secular courts. The dayanim are able to take into account local law and minhag hamedinah and rule within the guidelines on Halacha. The courts have their own system of ruling which is often against Torah law. MInhag hamedina does not mean we replace Torah law with secular law. Rather that Torah law takes into account the accepted norms and practices of society with regards to money dealings.