Should the UX of a website be treated differently and if so, how, when it comes to the difference between someone shopping for an e-commerce "buy-now" consumer product e.g. t-shirt, cellphone case etc., versus a B2B sell for a much larger ticket item, e.g. $500,000 (USD) building or engineered pump station for industrial plant?
Yes, there are major differences between cheap consumer products and expensive business purchases. The things that customers care about differ greatly. Also, customers are going to spend a lot more time deciding about an expensive item, so providing extensive information about the product is more important than rushing the customer through the process. For this reason, many B2B websites will just ask you for your phone number and get you talking to a sales rep right away.
The most important consideration, however, is that the website emphasizes the selling points that are relevant to the product. For example, customers for expensive business items are likely to care about:
Compared to cheap consumer items:
ISO 9241-210 says user experience is "a person's perceptions and responses that result from the use or anticipated use of a product, system or service". It includes all the users' beliefs, perceptions, preferences, emotions, physical/psychological responses that happen before, during and after use. (See also UX on Wikipedia)
Given that B2B decisions are also taken by humans, your website UX needs to be whatever it takes to make those humans feel good about starting a business relationship with you. That includes careful, and usable website design and presentation, attractive and convincing product descriptions and presentations, usable sales and backend processes and experiences, and so on. In a nutshell, the feelings you want to evoke are different between B2C and B2B, but UX has the same importance in both of them.