I took a quick look at your exchange and one of the things I have noticed is that you use bitcoin legacy addresses for deposits. It's recommended to use segwit addresses not only because the cheaper transaction fee but also because using legacy makes your service looks outdated.
I also noticed that the provided btc address isn't a new one and it has many transactions! For privacy reasons, it's advisable to provide a unique address for each order.
The exchange does not support segwit format yet, we may add it in the future so users can pay less network fees.
Concerning the addresses we use those of the exchanges so yes the addresses may have already been used.
It's like most cryptocurrency exchanges available on the market.
Thank you for your advice 🙂 we will take note of it ✍🏻.