Steve Marcus/ReutersBlackRock's Rick Rieder says there are "no sellers" in the equity market.Stocks will keep rising despite soaring market valuations, he told Yahoo Finance."If there's no egregious piece of news, geopolitical, et cetera, the natural migration is to higher prices."Even with stock-market valuations hovering near generational highs, BlackRock's bond chief highlighted one key reason why stocks can continue higher."There's no sellers," Rick Rieder — the firm's global CIO of fixed income — said at a Yahoo Finance conference.On the buying side, momentum is abundant, even as the prospect of persistently high interest rates boosts the appeal of the money-market funds."You think about the amount of money that comes into 401(k)s, wealth, salary — X percent goes into equities, and there's no sellers," he said.Rieder added that corporate buybacks are also backstopping stock market prices.