Can a composition dealer sell goods not taxable under GST

Yes u can sell goods even it not taxable .

RESTRICTIONS & CONDITIONS OF COMPOSITION SCHEME DEALERS :

As per the Provision of Section 10(2) clause (b) of the CGST Act 2017, A person registered under Composition Scheme will not be allowed to sell any "Non-Taxable Supply of Goods" ( like Alcoholic liquor for Human Consumption, 5 Petroleum Products etc ).

However they are eligible to Supply Exempted Supply.

Note that Exempted Supply and Non-Taxable Supply are different.