Configure yum to install private builded rpm

When you point yum to a repository it doesn't traverse the repository in search of *.rpm files, it initially only tries to access the repository meta data.

Those meta data files provide yum with the overview of what packages are available in that repo (and more) and make yum operations much more expedient.

After making changes to an existing repo or when you create a new repo for your custom RPM files, you need update/generate the repo meta data, because otherwise yum won't detect those changes.

You need to install the createrepo package and then use the identical createrepo command for that.

cd /mnt/old_valid/BaseOS
createrepo --database /mnt/old_valid/BaseOS

Additional flags like the --update can speed up createrepo operations on existing repo's where only few changes were made.