Can you not use one of the existing eCommerce solutions? See: http://www.3rdparty.info/products_ecom.html

There are also existing integration options see: http://www.3rdparty.info/products_int.html

Building a new integration like that from scratch would be a huge job.You wouldn't really need the SDK (which is for developing internal sage 300 modules). You would use the API which is available to any programmer. But I'm an SDK developer, and don't have much experience with the API, so can't tell you much more than that. See: https://smist08.wordpress.com/2013/10/12/an-introduction-to-the-sage-300-erp-net-api/

Hi dingsoft, thank you for your reply.  My understanding is that in order to do the integration, we need to build a module on the Sage side, and then a connector on the website's side. We would need to utilize the SDK for the module we need to build on Sage side. Is this correct?

FYI, the Sage SDK is not required, you can use any COM-enabled language, like C#, VBDotNet.

Thank you! Yes, we plan to use C#

You should have a look at Realisabel IMan to integrate.  80% of the work is probably already done for you. 

You should have a look at Realisabel IMan to integrate.  80% of the work is probably already done for you.