Hello @RolyPoly,
As @Sparkling_Juice has said, report this to Gameflip Support, and in future, always comment on a listing asking if the person is online, unless the item is instant/bot.
I too use the new online tag, although it is annoying that it can only be maximised to 4 hours. It should be extended to a time that the seller can use.
@mastercadem, I think that blocking people is at the sellers discretion. If they choose to block someone, they can. Also contact support to make sure this is correct.