It's not an easy issue to deal with, as whichever way you go you're going to get up someone's nose
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Around the audio recording forums I've frequented for +12 yrs, the general "rule" is that if any person is offering for sale items which they've sourced from a "manufacturer" or "manufacturer's agent", or offering modded equipment on a regular basis then they are selling commercially.......no ifs or buts!
The only "exception" is when someone has organised a "group buy" where forum members, in good faith, indicate interest and/or place an order for X number of widgets. Money is transfered to the "organiser" and once the prearranged total widgets needed to deal with the manufacturer is reached, the "group buy" is closed and the order placed. The "organiser" handles all aspects of the "buy" right through to despatching the widgets to the members who are involved and in most cases the "organiser" doesn't make much if any money for his efforts, the idea is to help out forum members NOT make a profit.
"Vendors" who use the forum classifieds to source gear are another story...........twice since joining IIS I have been preparing (convincing the minister of finances that I needed to spend money) to contact a seller for the same specific item, only to find that on both occasions the same member got in first, a little bit of digging around would indicate that it's unlikely he was buying for his own use.