Oh dear. Your tank isn't cycled at all. The fact that it has been run has no bearing on the cycling process. The beneficial bacteria that keeps your tank cycled can only live a few hours out of water. I'm guessing the filter from the tank has been out of water for at least a day?
You basically have two options.
1. Take your fish back, and do a
Fishless Cycle, and then get fish once your tank is cycled.
2. Cycle with the fish in there. You need to be careful with this, and do water changes frequently so the levels don't get dangerous for the fish. I still doubt the Guppies will live through it.
I'd personally go for the fishless cycle. However, it's up to you. If you look around, or search, there are tons of topics on cycling. I'd suggest you read through those. The Beginner FAQs at the top of the forums are also great.
Good luck with your tank!