You have obviously chosen to relist the item, for a much lower price, so it makes sense someone else is going to buy it if they want it. 

For your original buyer, you said you would send them a postage quote on Monday, so of course they aren't going to pay before that. It's not midday yet, they are probably at work and may not be able to access ebay until they get home. They bought 2 days ago. They have 4 days to pay before you can open a non payment dispute. Yet, the very next day after sale, you relisted the item.

Where do you go from here? You send the postage quote to the initial buyer via an invoice, and give them time to pay. If they don't pay within 4 days of sale, you can then opt to send it to the second buyer. If the first buyer pays, you cancel the sale for the second buyer and wear the well deserved negative feedback that you'll receive. 

Regardless, you are going to have to cancel at least one of the sales to get your fees back, or you'll be paying way more than you should be. In future, don't relist a sold item. You have sold 125 items, you should know what you are doing by now.