How do I Use a list

So I have A logs and money variable. Every 0.5 seconds I want 1 log to be subtracted and 50 dollars added if the selection on the list is "Sell logs". For some reason it won't do It every .5 seconds, how do I fix this? The actually programming I have

Every 0.5 seconds subtract 1 from Wood

Selected item=0 Add 50 to Dollars