Having begun to understand the logic of the Ebay catagories the main issue is the subdivision of OO locomotives, coaches and wagons. These issues may apply to other scales also however OO is the one I am most familiar with.
There are currently three subdivisions which are Hornby Dublo, Lima and other.
Other is clearly a massive catagory and within this you would under the new system place Hornby, Bachmann, Mainline, Airfix, Wrenn, Triang, etc.
Its really a question of whether it is acceptable to place all these collectable brands into "other".
The only other real issue is the area where items are suitable for both HO and OO. Without going into any scale issues in this thread where do you put track and scenery? If its clearly something for British OO modellers (eg Hornby, Peco track and buildings) then you would plump for OO. If its clearly something that appeals to continental HO modellers (eg. Marklin, Fleischmann track and Faller buildings) you would plump for HO. Figures and scenic items become less clear.
Ebay no doubt have had feedback on these points.