i used joomla and virtuemart for a shop recently - i like joomla and cant complain about virtuemart except that if you want to tweak the style to get it the way you want ( proper integration as opposed to use out of the box - and out of the box it isnt exactly the most beautiful cart i've seen ! ) then it can be hard to find out how to do do certain things - in saying that i still did get it the way i wanted in the end and there is a virtuemart forum which helps. i will be using virtuemart for all future shops unless a client wants otherwise. i found it very easy to use from the client's point of view ( for stocking shop making discounts etc) and it does everything you'd expect it to - also i was able to get it to into kilos and euros without too much hassle.
cant really speak for it as regards
seo but joomla can be put into search engine friendly mode. AFAIK a lot of
seo stuff for shops is in the wording of descriptions and the like - like titling an image u2-music-cd.jpg as opposed to cd8976876.jpg ( and that sort of thing ). but i think that virtuemart urls are friendly as long as you have joomla tweaked to SEF mode