Sold item on ebay but haven't received payment?

i sold a bunch of sega master system games on ebay on thursday and i haven't received any payment from the buyer yet, i sent him an invoice with the amount due but i haven't heard nothing back since, i know it can sometimes take a while but i was just wandering what should i do if i don't receive any payment and how would i go about doing it. i've sold items before and haven't had to send an invoice to get payment, payment was instantaneous, why is it different this time. thanks