NoSQL Database for ECommerce
Since e-commerce can encompass everything from shopping carts through to membership and recurring subscriptions, it is hard to guess exactly what requirements and complexity you are envisioning.
When constructing an e-commerce site, one of the early considerations should be investigating whether there is already an established e-commerce product or toolkit that could meet your requirements. There are many subtleties to processes like ordering, invoicing, payments, products, and customer relationships even when your use case appears to be straightforward. It may also be possible to separate your application into the "catalog management" aspects (possibly more custom) versus the "billing" (potentially third party, perhaps even via a hosted billing/payment API).
Another consideration should be who you are developing the e-commerce site for: is this to scratch your own itch, or for a client? Time, budget, and features for a custom build can be difficult to estimate and schedule .. and a niche choice of technology may make it difficult to find/hire additional development expertise.
A third consideration is what your language(s) of choice are for developing your application. Some languages will have more complete/mature/documented drivers and/or framework abstractions for the different databases.
That said, writing an e-commerce system appears to be a rite of passage for many developers ;-).
For MongoDB in particular, you may want to look into:
Forward - a new open source e-commerce platform using MongoDB (with declared intent to support other databases). Currently described as in "private beta" but looks worth investigating (and apparently is used for a few live sites already). I noticed it was recently mentioned on the MongoDB blog: How MongoDB makes custom e-commerce easy.
MongoDB and E-Commerce - a blog post from Kyle Banker, author of the Manning book MongoDB In Action. It's a few years old, but has some interesting discussion on data modelling considerations; there was also a follow-up on E-Commerce Inventory. Note: the aggregation and reporting options available for MongoDB have greatly improved since those posts.
Perform Two-Phase Commits - a design pattern for doing multi-document updates in MongoDB
The Master-Master replication (MVCC) you mention doesn't seem like it would be a relevant feature for e-commerce, where you would generally need strong consistency rather than eventual consistency. You mention syncing user databases, so perhaps that specific requirement might be better addressed by a single sign-on solution like OpenID.
MongoDB 4 now multi-document ACID transactions! That makes it suitable for e-Commerce!
Check out: https://www.mongodb.com/transactions