Actually, the default SRCDIR is /var/cache/sbopkg (which is where downloaded source files are kept to prevent the need to re-download) while the default OUTPUT is /tmp (where built packages end up so you don't have to rebuild them). These files *can* be deleted. But, before doing that, I'd delete the stuff in the default TMP dir, which is /tmp/SBo, which is just where the files produced by untarring, building, and pre-packaging are kept. Since these all come from the source and end up in the package, they're redundant, though they can be handy to inspect. This is why, if you want them deleted automatically, sbopkg provides the CLEANUP configuration option.
On the other hand, /var/lib/sbopkg/SBo/13.1/ is your default local repo and is the mirror of SBo and only takes about 90 megs and should never be deleted or you'll simply have to resync the 90 megs of files before you can use sbopkg again.
All of this and more is documented in the lovingly hand-crafted sbopkg.conf(5).