I created a web store because it offered 2 usability advantages over forum thread. I'll explain:
1. Improved customer usability - Multiple Pages: In a forum thread, you can't have multiple pages. As you can see, all new store makers naturally have separated their items into several tabs (e.g. Ult, Sup, Acc, Weapons, Pets, Etc). A forum thread essentially forces all of these pages into one giant, unwieldy, monolithic page, which is not user friendly and not scalable. Consequently, with pages, you can also add a new one with announcements and such without cluttering the other pages (e.g. April's auction page and my Free Items page that pointed to DDFR and advanced DD wiki info).
2. Improved seller usability - WYSIWYG Organization: Like you said, "it's easier for sellers to manage". Weebly is WYSIWYG which is easier than the forum's BB Code, although admittedly BB Code is not super complex. But when you have items numbering into the hundreds, you don't want to edit a 5 mile long page of code constantly. Weebly also offers nice visual features like changing SOLD items to black n white or adding "SOLD" overlays on them which others have done, enhancing customer usability. These are done with simple clicks of the mouse. A forum thread does not have enhancements like this.
I think a [WTS] thread is definitely still usable, especially if you only have a few items to sell. But if you have a large volume of items with a continuous resupply, or if you just want to create a more polished, user-friendly and easier-to-manage shopping experience, a web store can offer advantages as listed. As most major companies (Apple, Google, Amazon, etc) will attest to these days, usability and user-friendliness is important and I definitely credit some of my success to that. Competitive pricing was definitely helpful too of course :)