Back in the days when Britain was still Great, I could buy a pair of shoes, size 8 knowing they'd fit.
Then came the Euro with their size 42 which is almost, but not quite, a size 8 and varies considerably, I also find European shoes tend to be narrower than UK shoes.
Next we have American shoes which are ½ - 1 size larger than ours and men's and women's vary by two sizes, so I could take an 8.5, a 9 or even a 10.
Now we find Asian sizes which are loosely based upon the centimetric size of your foot, rough equivalents being 26.5/265 or 27.0/270.
All the above are made worse due to varying quality standards. Even now, UK shoes which used to be the same for male and/or female vary by up to a whole size, thus any shoe which can be considered unisex is yet another factor to consider.
The long and short of it is that buying shoes without first trying them on is a total gamble. There being fewer and fewer "proper" shoe shops in the high street, buying any shoe whatsoever is a nightmare!