Learn English – the word to describe a situation when rival gangs come together to figure out who is stronger

The seller has been very considerate; he has shown considerateness or consideration.

considerate
adjective con·sid·er·ate \kən-ˈsi-d(ə-)rət\
: thinking about the rights and feelings of other people : showing kindness toward other people [m-w.com]

consideration
noun con·sid·er·ation \kən-ˌsi-də-ˈrā-shən\
: careful thought : the act of thinking carefully about something you will make a decision about

: a desire to avoid doing something that will make another person sad, upset, angry, etc. [m-w.com]

As @talmu pointed out in a comment, it would be more acceptable/idiomatic for the given situation to say something like "Thank you for being so considerate."

fractious

1(typically of children) irritable and quarrelsome: ‘they fight and squabble like fractious children’

1.1 (of a group or organization) difficult to control; unruly: ‘King Malcolm struggled to unite his fractious kingdom’

‘After going backwards at the election and losing ground in opinion polls since, Opposition MPs are cranky, fractious and looking for answers.’

Oxford Living Dictionaries, Fractious. https://en.oxforddictionaries.com/definition/fractious

Fractious can describe individual people, but more often describes a group or an organization.