As demand for Apple's new iPhone heats up, some people that have received their pre-ordered iPhone 6 and iPhone 6 Plus are listing the devices on eBay to make a quick profit.

The highest listed price for the iPhone 6 Plus was $6,000, though there were many more options from other sellers that were asking for much lower than that figure for the smartphone.

According to reports, as of the afternoon of Sept. 12, there were about 30 iPhone 6 Plus listings on eBay that had a "Buy it Now" price of between $2,000 and $2,999, almost 300 listings between $1,000 and $1,999, and a handful that were below $1,000.

The "Buy it Now" feature of the auction site allows eBay users to sidestep all the bidding procedures and outright purchase the item for the listed price. 

A $1,500 "Buy it Now" price for the iPhone 6 Plus was popular among the sellers.

An "unlocked" iPhone 6 Plus, that is a device that does not come with a two-year contract with a carrier, has a price of $749 for the 16GB version and $949 for the 128GB version.

This means that a price of $1,500 for an unlocked 16GB iPhone 6 Plus represents a 100 percent markup, which is already a pretty good profit for selling off a device now that can be re-purchased for its standard retail price after some waiting time.

Sellers of the iPhone 6 and iPhone 6 Plus started increasing their prices after inventories of the devices quickly ran out when Apple began taking pre-orders. Within a span of hours, the shipping delay for an iPhone 6 Plus had increased to up to four weeks of waiting time.

For the sellers that were selling devices that they don't yet have, such as the iPhone 6 Plus, the listings came with the guarantee that once they receive the device, they would immediately ship the device to the buyer.

Purchasing the new iPhones online is much more convenient than falling in line in Apple stores for the Sept. 19 on-sale date for the smartphones. Individuals that were successfully able to pre-order for the iPhones online were promised a delivery date of Sept. 19.

Demand for the new iPhones is currently still outpacing the supply for the devices, which has Apple scrambling to replenish its stocks.

"Due to high demand, your Apple product may not be available for immediate shipping," said Apple on its online store. "Once we are able to complete your order, you will receive a notification when your product has shipped."