First of all, let's clear some aspects about the "purchase" ... Are you using Fotomoto? ... or perhaps you have some manual system for purchasing photos on your website? Normally, the pictures you have up for viewing should be optimized 100% in regards to viewing from the web, and then you should have much larger high-res images available to provide for the purchase. This is in fact how Fotomoto works -> All images are viewed at a normal web size, but when someone makes a purchase, you will be requested to upload the high-res version of the image. For example something like this:
Web: 1024px or 1280px long images, is a good compromise for the web.
Download/Purchase: Offering the full original size for those who purchase is the norm. Having your images available at 4000px or higher, and usually several MB file size. These files are not optimal to have displayed directly in the gallery though, simply because they take too long to load, too mumch memory, and don't have any benefit since they must scale down anyway.
So, unless I missed some details, for your main gallery you should simply focus on having a balanced optimal size suited in general for the web. I would recommend 1280 px long size ... This is nice and big for modern devices and internet connections. Some related post:
viewtopic.php?p=32569#p32569
viewtopic.php?p=22271#p22271 (... but from 2010, and the times are changing)