Some good points there minnapple. To expand on that, some I have worked with and I lean to flexibility in searh engine strategies.
Full eCommerce solutions are quite good nowadays and really easy to setup. Additionally, "rank wise" they will perform as well or better than static pages with buy buttons. In general, as Content Mangement systems (CMS) you can't go wrong and most are expandable which is a major factor to virtual stores and change to new solution when more successes or larger cost far more than just the upfront cost.
Expansion is a big time saver for inventory/drop shipping and many plugins, as opposed to drop & change, start over approach. Having known and optimized for many cart systems on the market they each have pluses & minuses... my general comments on a few.
OSCommerce [oscommerce.com] is a good product if you are savvy in open source (it's also free software) and the "open" part means it is easy to expand yourself (and/or cheaper to outsource this expansion). While it doesn't come with technical support it is used by a growing market and a forum is available and populated by current users where any problems you may have are likely answered by others that have worked through it themselves.
CAVEAT: A client; a mother of two, not techo savvy knows OSCommerce inside out so the learning curve isn't really a big one.
While OSCommerce is districtly php, it is highly recommended if your sales are primarily Search Engine based as this cart is extremely search engine friendly. It also can be made W3C compliant.
Store2003 [volusion.com] Renamed Volusion is another eCommerce cart with free setup (but month fees for invstory size (believe it starts at $100 US). This cart allows html page generation which is often a concern for virtual business people - but major search engine don't have a problem with dynamic .php databases.
While toll free 24 hour tech support exists, I think the owners under-estimated their success as you can almost never get in contact with them but the cart is also extremely search engine friendly. It also can be made W3C compliant.
Also all things being equal - if you intend doing your own SEO - this is likely the best option.
X-cart [x-cart.com] has support (but licensed software) so you pay for it (I believe it starts at $400, plus monthly support fees. While it also includes html generation like Volusion there is a small problem in that the top link hierarchy is always in php thus the breadcrumb trails leading back to mainpage are .php - so while a search engines and visitor start out at html pages - the hole in the top level at some point switches navigation to php (and you can't get back to html).
I say "small" problem simply because you don't need the html conversion for search results listings. The greatest value though that html generation offers to owner is the fact you can use this html pages extremely effectively to sagregate PPC traffic (robots.txt excludes stop the dup content)... particualrly useful if you don't deploy a robust tracking solution.
I will stress - it best not to use html generation if ranked results are your primary concern as search engines do not get a coherent link construct in this format. Clients are quite happy with the support provided with X-cart - in fact it appears to be the only operations you don't need to wait of anything and and staff are easy to work with.
X-cart appears to be a composite of OSCommerce open source software and modified for licensing (not 100% sure of that - but that code appeears quite similar.
It is interesting to note this company is Russian but to date zero problems have developed over language barriers (written or verbal), cultural differences, or technology concerns, and they seem to respect every client - they impressed me (Support to SEO)- even though I don't actually use their solution. X-cart can also be W3C compliant. (most don't concern this a factor but my preference -- it is).
MonsterCommerce [monstercommerce.com] to be fair I highly recommended this cart to clients early based on the apparent spotless reputation.
Be that as it may... I was surprise to find that users of MonsterCommerce do not gain access to their own FTP account -- you give this up to them on entry. This kills many alternatives to page locating/relocation (mod-rewrites and server side re-directs), and many other SEO considerations... unless MC does it themselves - and as last known they don't.
Additionally, since you don't have full access to proprietary code you can't fix all W3C Compliancy errors. Setup is is free, and less than 5K inventory is $80 and quite affordable and luckly the greates advantgae of MC is a drag & drop, ready to go off the shelf solution, so an ideal solution for those the have virtually limited in web code.
For SEO reanked results - well these solutions can rank - but tend to require twice the general SEO resources (best practices) of of all the above to hit the same rank targets.
Support & customer service - well from my experience "great" early on... but this appears to be MC weakest area and much improvement can be made.
It's worth noting - if you use WorldPay, MC did have a problem with this (don't know if still a problem - but you should check in advance).
There are tons of other solutions and custom/scalable ones as minnapple pointed "are" the way to go for growth strategies but can be costly and you really need to do your homework so you don't purchase a lemon - in hindsight.
A closing comment - the general website design of all the above solutions tend to be equal... a good professional looking shopping cart has little to do with the solution itself - all can be 100% customize for feel and market appeal.
Someone can correct me here if I am wrong but all of the above solutions are world currency compatible - most through plugins.