HEllo question about the 72hour transit visa

i bought a ticket from Manchester to beijing and beijing to korea seoul, but this ticket was bought in agent site so i cant select the seat yeat. it can be selected just 48hours before the plane, but i will arrive at 25th in beijing and the plan from beijing to seoul is 27th afternoon, so when i arrive at the beijing airport i have a e-ticket but the seat is not selected is it ok ? because i saw some people say the seat must by selected