Almost all mochrei stam I know sell pshutim mehudarim tefillin (batim). Many of these vendors try to keep a minimunm standard of Kosher lechatchillah, including myself. While the parshiyos they put into the pshutim mehudarim batim may be kosher lechatchillah, are the batim (and consequently the finished pair of tefillin) kosher lechatchillah?
I have asked numerous poskim this question. They are certainly not mehudar. And the old style pshutim are definitely bedieved. But some poskim seem to be reluctant to call pshutim mehudarim kosher lechatchillah, because the bayis is glued at the edges.
On the other hand many of the older style pre- war batim, worn by may chosheve yidden (including notable rabbonim) were essentially made in the same way as pshutim mehudarim are made today. Furthermore pshutim mehudarim have some advantages over cheap commercial gassos that have all sorts of serious problems such as false charitzim, tiach, holes etc, problems which which do not exist in opshutim mehudarim.
I think therefore it is acceptable to sell good quality pshutim mehudarim as a basic pair of tefillin, provided that the buyer understands that they are only entry level and that the parshiyos are decent and kosher lechatchillah leksivah.
Furthermore, one should try to buy only pshutim mehudarim with a)"knafayim", b)mechitzos that come down to the titurah and c)the outer shin on the shel rosh which comes down to the titurah. Without these three features, it is very hard to say they are not definitely bedieved.