In the eBay Printer and Receipt preferences, you need to select the Zebra LP2844 format to print 4"x6" labels.
In the Dymo printer preferences from the Control Panel you also want to select the 1744907 label.
There is nothing in the browser you need to select. However, you want to make sure the browser is opening PDF files with Adobe and not using its built in reader.
When the Adobe label preview opens, check the Target Printer is set to the 4XL, Size is Actual, and Orientation is Auto.
You will have to create a new label to test this, as you can not use a previous label created in some other size format.
If still a problem, come back with details. A screen shot of the label preview will also help.