I have clients on old ecommerce platforms such as OSCommerce and ZenCart that I would LOVE to move over to WordPress (have you tried to SEO an OSCommerce or ZenCart site? It can be done but it’s like three pains in each ass cheek.)
I have a full developers license for Cart66, which I can use for a couple of personal projects, but it doesn’t support tiered pricing. This is a basic requirement of ecommerce, and it’s positively ridiculous that they don’t support it. Moreover, you can’t find out they don’t support it until after you buy it – the support forums for Cart66 are full of people who have been begging desperately for this feature for over two years – but the forums are only open to people who’ve bought the product. Megafail.