As can be seen from Sugar's link I was in the same place as you some 5 months ago.  Our partnership is 20+ years and we have been on ebay as long as it has been in Australia.  We were also required to provide a "Letter of Authority" for the other partner and had trouble uploading it.  Never could get it to work on Firefox but did eventually on Chrome, also required several other Stat Decs.  Not sure if its still available but I dealt with the Management Payments Team directly, but I can't find the link any longer.  The woman I spoke was actually quite helpful and knew her stuff.

As an aside after 5 months on MP, funds were suddenly put on hold, and I had to reverify the Bank Account that eBay has been sending MP funds to for the last 4 months.  From the start eBay have handled this appallingly, its not like Partnerships, and Trusts are some new fangled untried financial instrument.