The trouble is that i am used to record collector grading / values where vg is valued at 50 % of mint , ex 75 % of mint , if a seller uses vg as his 2nd grade it doesnt always register as i dont go through every sellers listing to see the grade ruling they use.
If i see a £50 book priced 45 for sale in vg i automatically think that the guide price for this is £25 , therefore i am not paying top guide price for a 45 in lower condition even if it using a different system