Pulkit has written a wonderful post on dealing with unregistered person under GST. You should read that article to understand the full consequences of not registering under GST.
However to make it clear first you should understand why a person has not registered under GST.
If his turnover has not exceeded 20 lakhs in a year then he not a defaulter under the law, however if turnover has crossed 20 lakhs and he becomes liable for registration then he has to get himself registered.
Few disadvantages to an unregistered person are:
- Registered persons will not trade with unregistered persons due to reverse charging of tax and negative impact on cash flow
- Can not bid for high value deals as most of the companies will require GST number
- Can sustain only in B2C transactions
- Cannot avail input tax credit