I presume that few, if any, trademarks (at least US) have been granted for cannabis strains, with any granted ones somehow slipping by the requirement that trademarks only be granted for legal products/services. Old and names long used commercially, such as Sour Diesel, do not receive trademarks. So in nearly all cases, commercially using a strain name used by someone else pretty much comes down to asking that strain's source nicely for permission (or not), no legal requirements or restrictions on others using non-trademarked names, only what personal/professional ethics demand.