On the Ubercart e-commerce solution that we typically use, there are many, many different options to handle different types of needs, such as:
Collecting membership dues and assigning appropriate permissions and roles based on the product purchase
Selling products from a single store
Selling products in a marketplace situation, with multiple sellers
Offering a "catalog" of items
Selling event tickets
Providing digital downloads
There are many options within the available "universe" of tools. Here are some tips that we've used to great effect:
1) Integrating Follow-ups
If your customer is in the process of purchasing something, and then decides against fulfilling the order so that the cart is now "inactive" or "abandoned," it's possible to set up the "Follow-ups" module to send a reminder e-mail to that person within a certain time frame. We use this with a coupon and/or a gentle prompt with a customer service phone number if the customer is having issues. This e-mail typically encourages the user to complete their chectkout.
This can also be used as a follow-up prompting a product review, or some other action that you'd like the customer to take, if the followup is sent within a certain time frame (such as 100 hours after an order is marked as shipped).
2) Offering discount coupons
The coupon code is a time-honored way to encourage your followers to peruse the catalog. Our clients offer codes to their Facebook or Twitter or Pinterest users, driving them back to the website to purchase a product or service.
3) Assigning a role upon purchase
For membership organizations, being able to track a user's status can be made more automatic by offering a certain role for a certain time frame (for example, "paid member status for 365 days") after the purchase is made. There is an automatic followup that can be sent to the member within, say, two weeks of their expiration date. By using "views" of the data to figure out who is due to follow up, or who is recently expired, you can retain your general membership and increase the automation of collecting dues.
4) Digital download upon purchase
It's possible to offer a digital purchase, such as a music track, a set of images, or a white paper or report. By offering the digital download, when a customer checks out, they receive a unique e-mailed link to download a copy of the file. The link is valid for a certain number of downloads and/or must be used within a certain time frame. The link expires after the download is complete.
5) The "Marketplace"
The Ubercart Marketplace module allows a store owner to offer "sub-classes" of products where other storekeepers may sell products within the e-store, with a potential for a commission to be charged on any sale originated in that marketplace. The Marketplace module offers a powerful way to aggregate multiple sellers' products into one catalog and automate the process of notifications and checkout in exchange for a variable fee, such as a fee charged to set up a storefront and/or a fee charged on every purchase.
6) Affiliate sales
With affiliate marketing, an affiliate may use their own code and generate a link to a product on your store, gaining a percentage of any sales that originate through that link. Ubercart Affiliates offers a way for the store owner to control, report on, and manage affiliate-generated sales and commissions, on both physical products and digital downloads.
7) Shipping, taxes, additional fees
It's possible to configure your store to match your business-specific needs regarding shipping, taxes, or other fees. For example, California-based companies add extra "Rules" to collect tax based on the customer's city, based on Board of Equalization requirements.
In general, the Ubercart solution is powerful enough, and elegant enough, to cover 95% of our customers who need e-commerce.