Nvidia's $100 billion investment in OpenAI has investors questioning whether we're seeing too much circular investing: companies using their free cash flow to give their biggest customers money that will make a round trip back home.Why it matters: Wall Street is already concerned about an AI bubble, and this kind of funding just adds fuel to the froth fire.Catch up quick: Nvidia's OpenAI investment is meant to support data center buildouts.These data centers will likely run on "millions" of Nvidia chips, according to a statement from the company.Nvidia told the research firm Bernstein that its investment in OpenAI would not be used for any "direct purchases" of Nvidia products.Zoom out: It's not just about Nvidia. All the large-cap tech firms are jumping on the circular financing train.Meta is in talks with Oracle about a $20 billion cloud computing deal, while Microsoft is pursuing a similar deal.Microsoft, Meta, Amazon, Alphabet and Tesla account for over 40% of Nvidia's revenue as they buy Nvidia's AI chips.The highly interconnected nature of the Big Tech names has Wall Street concerned about how much the market could drop if the AI bubble pops.What they're saying: "It's kind of like having your parents co-sign on your first mortgage," Jay Goldberg, an analyst with Seaport Global Securities, said on Bloomberg Television, adding that the deal is "bubble-like."Goldberg is one of Wall Street's biggest Nvidia bears. He has the lowest price target on the stock among analysts tracked by Bloomberg, at $100 a share, 43% below the current Nvidia share price.Yes, but: Other analysts tell Axios that circular investing is just how public companies and markets work. "I don't get why that is a problem," says Max Kettner, chief multi-asset strategist at HSBC. "That's the nature of business."If Nvidia is paying its customers to "just buy the AI capex," Kettner tells Axios, "in three years, once it's gone too high, too far, cool. You stop."Flashback: Previous examples of vendor financing produced some "horror stories," according to a recent note from investor and Capital Gains substack author Bryne Hobart.Cisco in the 1990s: Lent money to telecom equipment companies because buildouts were expensive and demand was huge. This led to an overbuild, and Cisco's stock has been tarnished by the dot-com bust since. Hobart noted that this amounted to charitable donations for streaming platforms using this bandwidth later on.The lesson? Suppliers can juice demand by lending, but there are overshoot risks. The hope? That it's different this time.Threat level: Knowing when to stop buying and even sell is obviously the key question for investors who've been happy to ride this AI rally straight to the bank.One sell signal would be "an earning season where margins suddenly surprised to the downside by 1 or 2 percentage points," HSBC's Kettner says.To him, the bigger risk is not riding the rally rather than realizing a little too late that it's over.What we're watching: Earnings estimates, which have not hit unbeatable levels just yet.If estimates outpace results, that could be the Kettner signal to sell.