See also bug 18187. This field in items is labeled as Source of acquisition. Since it is a free text field, we can imagine that any information may be entered into this field. You could e.g. also enter a name of someone who donated a book. Unfortunately its literal name is booksellerid. Originally, it may have been the intention to really save the booksellerid; but making it an editable text field was a bad choice. Note: MARC21 maps 952$e to booksellerid, French UNIMARC maps 995a to it. Would you agree with renaming the field to acqsource?
At the moment we use it in both ways in our library. Libraries not using the acquisitions module use it as a free text field adding the name of the vendor there. In libraries using the acquisitions module the booksellerid of the vendor is added by the acquisitions module. I am not sure what we will win by renaming it?