Is helping petitioners create accounts on social media websites (such as Twitter and Instagram) or buying and selling websites (such as eBay) considered a sin if these accounts are used to disobey Allah, and is it obligatory to refrain from helping only those whose request is clearly for disobedience (such as asking for movie websites)?

The basic principle is that it is permissible to teach how to open accounts on permissible websites, except to someone who is known or strongly presumed to intend to use them for something forbidden. As for someone whose situation is unknown or doubtful, there is no harm in teaching them. However, it is forbidden to teach or guide to websites specifically dedicated to forbidden things, as this constitutes aiding in sin, and Allah has forbidden cooperation in sin and and aggression.