Why stay away from Flues models? 2 reasons come to mind. First, when Ithaca went from the Flues to the Knick model, it was a significant redesign internally, many improvements made. If you want details, I'd suggest checking out Walter Snyders excellent book. Second, and this applies especially to the grade 4 guns, the Knicks are considered more desireable than Flues by collectors.
The gun being sold is represented in being in pristine condition. If so, after having it checked out by a gunsmith familiar with Knicks, I'd encourage you to shoot it. They are all choked very tight, so they really crush the targets. Crushing targets with an 80+ year old gun is especially fun.
If you are looking for a Knick with even greater collector value, I'd suggest looking for a grade 5E, or if budget allows, the more rare 7E. Again, condition and originality of the gun have a strong determination on value.
Pete