I think this comment sums up perfectly what I wanted to reply myself.

In cases like this, the outcome is often situation-specific. Spells alone are no reason to exclude a sale if there are no other data points, just like any regular high sale is not automatically exempt from being used in a suggestion without the appropriate counter-evidence. They can however be indicators that a sale may be higher than others for the sake of the spell, and as such be cause for excluding the sale as an outlier.

In this particular case, I see two sales relatively close together, and one that sold for 50% more, which generally already implies that the higher sale may be the outlier. The fact that it is spelled may explain why the sale was so high (especially given the traders involved, I've seen Gibson buy more oddities such as spelled or very specific quality items that hold a collectors value for him), without that fact itself being the sole reason it is excluded. If that was the only sale, or if more sales would have backed up the data point, I'd have been fine including it. I'd also have vouched for a full range, had there been only 2 sales.

However, given that none of these apply to this situation, I agree that 749 is an outlier in this case and will be accepting this.