Although there are many conversion charts available on the internet, try google, none of them are all right. You will need to do your own research and get to know your own size well. If you are a seller on Ebay and want to be accurate in advising your customers you will need to burn some midnight oil on this one and not take any one size chart as gospel.
European(EU) sizes are the most consistent, being entirely measurement based and not effected by differences between mens and ladies sizes. Have your foot professionally measured in EU size, convert to your own known size and base your purchasing decisions on those sizes. This gives you two known points of reference when trying to read published conversion charts.
As a buyer there are some basic rules to follow, which will put you in the ball park:
Assuming you know what size shoe you wear in your own country/gender.
AUmens sizes are 2 sizes smaller than the equivalent AUladies size. (AU(m)8 /AU(l)10)
UKmens is the same as AUmens, UKlady is =UK and AUmen +1/2(most of the time) (UK(m)8/AU(m)8/UK(l)8.5/AU(l)10)
US conversion is a lottery as there are two different scales or lastes in use. The 'Standard' and 'Common' laste have 1/2 to 1 size difference and it doesn't appear to be shown on the shoe or the box, which laste was used. If you are lucky there will be an accurate EU measurement provided by the seller and you can use your foot measurement to decide if they will fit. US(l) = US(m)+2. US(m)=UK(m)+1/2 or 1 Given this, our example becomes even more approximate. (UK(m)8/AU(m)8/US(m)8.5to9.5/UK(l)8.5/AU(l)10/US(l)10.5to11.5) confused yet? Remember that these all vary from manufacturer to manufacturer as well.
EU sizes are based on foot measurement and one can only hope that it will be adopted world wide as seems to be happening. Having said that, due to the internal variations of manufacturers in other parts of the world, there can be no true equivalent. Everything is approximate. Have your foot measured!
Adding to this confusion is that of newer manufacturers who haven't done their homework either. As an example I am an AU(m) and UK(m) 8.5 and have been for 35 years now. Some, large number, of manufacturers, beginning about 20 years ago, made their shoes to fit me AU9. I can only guess that the UK laste for women was inadvertently adopted by somebody and has continued to be used by an ever increasing number of manufacturers. I read, recently, in a seemingly well researched paper, that the UK(m) & UK(l) size is the same!! Commerce is rapidly converting to online sales and if the footwear and clothing industry are to compete we need international standards yesterday. I vote EU.
Good luck and happy bidding.