I think it just depends on which seller's photos you are looking at.
For example, have a look at this (from my store): /www.hipstamp.com/listing/solomon-islands-1983-sg489-492-commonwealth-day-set-mnh/14212557
If you click on the image for the stamp set you will then see a full screen magnification.
Or if you hover the mouse pointer over the smaller image initially displayed you will see a light window of that portion of the stamp magnified. You can also use the scroll button on your mouse to make the magnification smaller or larger.
Have fun with displaying images here, very large images are supported and Mark has provided some great tools for manipulating them.