To be considered a letter, your item must:

• weigh less than 500g 
• contain flexible items**
• have a rectangular shape 
• be no larger than a B4 envelope (260mm x 360mm x 20mm) 
• be no thicker than 20mm

** Letters should not contain stiff objects such as bottle tops, pens, items enclosed in a hard case, and so on. High-speed letter sorting equipment could damage such objects, or the objects could damage other articles.

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Let's face it, if AP ever decided to enforce those rules, 99% of items currently being sent as large letter by eBay sellers would be rejected at the point of posting so it's pretty clear that while the regulation exists, it is certainly not being enforced. In any case, if there's any doubt, it's a simple matter to wrap the offending item in a small sheet of bubble-wrap which should prevent any of the hard edges that they appear to be worried about.

In addition, my large letters are specifically cancelled at the time of lodgement so I can photograph them for proof of posting, so they'll never see the inside of a letter sorting machine anyway.