This is a very simple warning - that many sellers wrongly describe their yarn as 'wool' when it's really a wool blend or just plain acrylic with no wool content at all. If you use an acrylic yarn in a pattern intended for wool you could be very disappointed; for example, the garment is more likely to stretch with wear (and especially when washed); the ribbing won't be as elastic and stretchy; and if you're planning a garment that needs pressing - as most fair isle and multi-colour garments do - you'll have a distorted fabric and all your work will be wasted.

I've noticed that some sellers list their yarn as wool but tell you in the text that it's an acrylic and wool blend. For example, many Aran weight yarns are 75% acrylic and 25% wool. These blends could knit a good plain garment if you keep to a firm tension. But some sellers list their yarn as wool and only reveal that it's a blend - or, even worse, that it's 100% acrylic - if you e-mail them with a specific question about yarn content. For some reason they regard the words 'wool' and 'yarn' as interchangeable. My personal view is that it's close to being fraudulent, especially when done by eBay shops - they should know better.

Price may be a guide, but my advice is: unless the seller actually states the wool content, ask before you buy and avoid disappointment.