I question the blanket 'no business advertising'.

While in some cases, this would be clearly inappropriate - for example -
'BUY MY AWESOME SCREEN PROTECTORS' - in others, it might be less so.

Stuff that you cannot easily find on ebay - for example - someone making replica 'flashing' fixtures - to access the debug pads of the phone, or a higher capacity battery for example.

Especially given there is a buy/sell forum, I fail to see why this should be forbidden.

Responding to users questions on a product you make is clearly a valid use of TMO, that should be encouraged.