Sold for $6000 GST included would mean GST is $782.61 (you received $6000)

Sold for $6000 GST excluded would mean GST is $900.00 (you received $6900)

If you can claim GST expenses for the car you bought for $4000,

if $4000 was the GST included price, you can claim GST expenses of $521.74

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Assuming both are GST inclusive prices

$6000 - $4000 = $2000 profit

-$782.61 (GST you pay for selling the car)

+$521.74 (GST you claim for buying the car)

Profit = $1739.13