It seems safari has been experiencing issues for quite some time in relation to "login cookies". They mutilate and/or delete them, which logs you out of ANY website (if you've wondered why, this is the reason).
If you do a quick google for "safari not staying logged in" you'll find it's a persistent problem which hasn't been fixed. Even on my personal iPhone 4S today, unless I click the "remember me" option I too get logged out.
After reading forums online it seems that even checking a sites "Remember Me" checkbox isn't enough to stop it from happening as a phone has a small amount of memory and it will just delete files from the cache at random and one of those may be a sites login cookie! VERY frustrating problem!!
If you experience this, you may need to switch to a browser on your computer (not safari though) and you will be fine.
Now may be a good idea to learn how to make a Used Solo Dresses iPhone APP!! :o)
Added in Seller FAQ on 31st Oct 2016