If every reseller proposed a discounted "no packaging" option, people would produce less waste. If providing such an option was mandatory or even just incentivized, more resellers would propose it.

Consumers do have some power, but they are not the only ones and we should demand an effort from all parts of society there.

On the other hand, I doubt it is plastic waste that produce methane, an organic byproduct. Maybe food wastes are at fault. In which case your enemy is not overpackaging and may actually be your (temporary) ally