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Why oil can only go lower from here: Gartman

Oil fell nearly 2 percent Tuesday morning after Monday's rally as the driver of crude's price shifted back to the oversupply of the commodity. And widely known investor Dennis Gartman says that if you bought the rally, now is the time to sell.

"It needed to have a bounce — it got a bounce," Gartman told CNBC's "Fast Money" on Monday. Oil traded as low as $40 on Monday — a key psychological level for the commodity — before staging a stunning reversal, closing 2.5 percent higher for the session.

"Anytime you see the markets trade at a continuously widening contango it tells you that there's plenty of crude oil around. Tells you there's plenty of supply of the underlying commodity. At this point the propensity to be a seller is strong. I think this will be a short-term rally."

Read MoreCrude eases as focus returns to global oversupply

Contango refers to when futures prices for a commodity are greater than the current spot price.

"As I like to say, crude is bidding for storage," added the editor and publisher of The Gartman Letter; meaning there's more than enough of the commodity to meet the demand.

For Gartman, crude is not a commodity worth touching at all right now. He believes any weakness in oil is not to be bought, and consequently, all rallies are to be sold.

While the focus of oil's volatility has been on WTI crude, less attention has been paid to Internationally traded Brent crude — the usual bellwether of unrest overseas.

Brent rose fractionally on Monday in comparison to WTI's rally.

"[The divergence] proves that the market's duty is to make fools of all of us," said Gartman. "One would have thought that if there was going to be a greater impact it should have been on Brent."