The limits for Nash Cash is based on your identity verification tier. You need Tier 1 status to use Nash Cash and can increase your limits by upgrading to Tier 2 or 3.

Here are the limits for Nash Cash. Note that these are different from your trading volume limits:

Tier 1: 25,000 EUR yearly buying limit and 25,000 EUR yearly selling limit.  
Tier 2: No buying / selling limit.
Tier 3: No buying / selling limit.