My GST liability (RCM?) If I purchase goods (FMCG) from Unregistered GST vendors?

No liability, unless those items which u buy are specifically notified as RCM applicable 

okay Do registered entity need Bill of Supply, self invoice Or any other document or just an entry while filing income ?

Also for eg If retailer buys from Gst reg entity eg at 5% gst

Cost of purchase from agency -152 rs
GST charges - 8 rs
Effective paid for purchasing -160 rs
Sale price - 180rs
Margin - 20 rs ( includes GST 5% =1rs)

Profit -19rs
Income tax at 10%

Tax paid 1.9

Earned (19–1.9) - 17.1 rs

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Now if bought from unregistered entity , who sells to me again at same price than registered supposedly again at 160

And I sell it forward for 180rs

Now I gained profit of 180–160 = 20rs

I paid income tax at 10% over 20

Earned 20-10% = 18

I earned more in this case ???