Street vendors are very blunt and against photographs of their merchandise. I learned the hard way after this particular vendor came rushing to me, yelling for me to delete my picture.
Of course I didn’t. Not out of disrespect, but out of an understanding that the artistic products he was selling were most likely duplicates or mass produced based on someone else’s work.
On the other hand, there’s many sweet and generous vendors who have been more than happy to allow me to snap photographs of them and their products.
Regardless, be wary and be careful when randomly photographing street vendors in Bangkok.