Danny,

Assuming you actually sell it, and incur no other costs in doing so, profit is (sale price - purchase cost) - this much is for sure.

% profit could mean profit as a percentage of sale price or of purchase cost. The context is important here.

The default seems to be for expressions like this to refer to numbers that "happen first", so if I had to guess with no other clues which one somebody meant I'd say

(profit/purchase cost) × 100%

But I would not be confident that that was what the person had intended, and if only the sale price had been mentioned: "I sell them for $15 and make 25% profit" I would suspect it was

(profit/sale price) × 100%

There is potential for confusion here - always try to make it clear which you mean.

Good Hunting!
RD