You may have some account customers who you invoice at regular intervals. These might be other dealers or businesses such as interior designers that you supply on a fairly regular basis. After account customers have been sent an invoice, but before they pay, they are your debtors. When they pay you, the payment is referred to as 'Cash from debtors'.
Make an estimate of the income you might receive from account customers and enter that amount in the month in which the money will be received.
Selling at auctions
If you sell goods at auctions the auctioneer will give you a statement showing the hammer price of the goods (what they sold for), the seller's commission you have to pay and the net amount due to you. This statement acts as an invoice from you to the auctioneer, who may not actually pay you until some weeks after the sale. Enter in your cash flow forecast any amounts you expect to receive from auction sales in the months you will receive payment.
Auctioning goods on eBay is different. You'll get paid by the buyer when your item sells - generally quite soon after the auction ends - and you'll pay eBay the listing fees and final value fee (commission) separately some time afterwards.