ShipScript has been an eBay Community volunteer since 2003, specializing in HTML, CSS, Scripts, Photos, Active Content, Technical Solutions, and online Seller Tools.
The viewport meta tag is unnecessary for plain text. The plain text will wrap just fine on a mobile device.
The viewport meta tag tells the mobile browser that the page does not need to be zoomed out or shrunken to see the whole thing, as would be the case for oversized pages.
Instead, the tag tells the mobile browser to not zoom at all, and to let all the content wrap (if it can). If we add the meta tag when the page does note wrap correctly, the browser will add scroll bars instead of shrinking the page down to fit into the available space. In my opinion, the scrollbars are worse than a shrunken page, but tolerable if the scroll is only the width of some borders and one doesn't really need to scroll to see anything of value.
We see a lot of templates that almost make it, but end up with some border padding that won't pass the mobile checker. I wouldn't worry too much about those unless some text is clipped.
While eBay's mobile checker is nifty, it does take some time to load, or sometimes fails to load entirely. You can visually see how your listings are doing by pasting your item number or your template code into the Sandbox tool and clicking one of the size buttons to see if scrollbars appear.
http://www.isdntek.com/ebaytools/ActiveContentSandbox.htm
Plain text should always be mobile friendly with one exception: If leaning on the spacebar is used to align text, or a row of characters like asterisks or underscores are used to draw a line, those are non-breaking elements that won't break in the middle to wrap, and that can cause the text to spread too wide and fail to wrap correctly.
HTML is not really a problem either unless there are dimensions that force the page too wide. For instance, the smallest mobile device eBay cares about is 320 pixels wide. If HTML is used to set content to 600 pixels wide, the page won't squeeze down or wrap smaller than that width. Or if HTML is used to make a table, the table contents may not allow the table to squeeze down to the small size.
Designers use tons of HTML to make mobile friendly templates that scale down just fine, so HTML itself is not the problem. But HTML that was written without mobile in mind is often an issue.
