I have a pet annoyance, particularly with the popular auction site, with regard to scale and gauge. People either try to purposely mislead, or do not understand that the terms "Scale" and "gauge" have different and specific meanings. Many post stuff and call it O scale when it is not. It is not so common the other way.
Scale is the term describing a model's proportion to the actual prototype. O Scale means the model is 1/48 the size of the actual prototype. It also means the scale is 1/4 inch of model representing 1 foot on the prototype in all dimensions. S-Scale is 1:64 and HO scale is 1:87 ( a discussion of why HO is 1:87 and not 1:96 is for another thread). These have specific meanings to modelers and define what the model is and what it will fit in with and look appropriate.
Gauge on the other hand refers the to distance between the running rails on the track. Gauge has no meaning as to what the model represents. It is simply means the model runs on a specific track width (between the rails) - such as O gauge, the track width, or gauge, is 1 1/4 inches. HO gauge track is approximately .65 inches between the rails.
Consider the Bachmann ON30 models. These are 1/4 inch scale, O Scale, models of narrow gauge equipment that run on HO gauge width track. They are not HO scale models! Another example is the stuff called G Gauge. Various manufacturers have made products ranging from 1:20 to 1:32 scale to run on this gauge track. If you are looking for a new passenger car for narrow gauge 1:22 scale trains and see one listed as a G Scale coach - what is it? Is 1:32 and way too large to fit with your layout, or is 1:22 and a perfect fit, or 1:20 and a close fit?
Now where this becomes very confusing to O scale modelers, is when people refer products from the major 3 rail O GAUGE manufacturers as "O Scale" I have seen old 6464 series boxcars listed as "O Scale" These are not O Scale! I don't know what scale they are - somewhere between O and S I think. It is obvious to most O Scale modelers in 2 or 3 rail, that these are not O scale. But for those of us who have been in 2 rail and away from 3 rail products for years, we often do not know if a product from these manufacturers, that we have an interest in, is scale or not when it is listed.