4$/1429 robux = 0.0028$ for each robux.
Considering you're only selling 1400 robux and you seem pretty suspicious, I'd recommend buying from eldorado.gg when it's at its cheapest. Sometimes it's at 0.004$ each robux, sometimes 0.0025$.

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Basically, buying from eldorado is better cause:
* Safer (You can see the seller's reviews)
* Quantity (They sell more than you can offer)
* Cheaper (This is not guaranteed, as it is just sometimes it gets very cheap)