...So you mean that _before_ implementing this system you could have bought, say, +5% accuracy 3 times, for instance (rasing it to 95% and adding 2-3% from item stats)? 

Sort of. Actually, the values used to be absolute. So, it would have added 85% on each time you purchased it as it has no basis to know the original value.

Now, I've made it relative so it technically adds 5% accuracy, and checks the purchase log to be sure that you haven't already bought it.