Why not use a script that is open source on github?
And if poloniex goes down/hacked you might loose all your btc's.
Besides that seems to me that you are providing a good service, as long as people don't check the 'withdraw' box on the API setup :)
edit: oh i read wrong, you are not the owner of the service
Correct. During API creation don't check any box, don't enable trade and withdraw.
Why not using script? Because I'm not good at commands.