the difference comes from the type of aluminum used and the amout of heat it can dissipate. as well as the number of fins in the cores. MOST of the ebay cores are manfuactured in China as knock-offs using cheaper aluminums that do not dissipate heat as well. My ex-buddies ebay core fell apart from sitting a winter and oxidizing, he took a regular hose to it and all the fins in the first 3 rows fell off.
There’s a reason it’s cheap and it’s not because of supply/demand. China is knocking off everything now, even certain well-known vendors are shipping OEM parts to china to have them duplicated.