Boycotts need communication, conditionality, plausibility,
1. Communication
I sell books.
If people boycott me and DO NOT tell me "PJ stop doing $thing and we'll start buying your books again."
Then I won't know WHY my sales are bad.
2. Conditionality
DON'T tell a company
"I'M NEVER BUYING FROM YOU AGAIN."
They shrug and say Ok.
I won't buy UNLESS/UNTIL this happens.
/1
3. Plausibility
I'm poor. If I write to Mercedes and tell them "I'm going to stop buying Mercedes until you do XYZ" then they will say "LOL OK". I'm not their customer, they don't care.
Some people might argue that you can really only boycott a company about their products but if you boycott say Sodastream, tell them that you were a customer and say you won't buy their stuff until Palestine is free, that motivates them to push for a free Palestine.
C.f. Boycott of apartheid South Africa.
@Homebrewandhacking somewhere there is a news story about a group of vegans organising a boycott of a North London butcher's shop because the shop refused to stop selling Foi Gras.
I'm sure there is. Focusing energy into useless activity and outrage is a primary activity for media outlets.
If you're participating in #BuyNothing then I'm saying you have to let them know that's what you're doing.