Qitzur Shulchan Arukh 62:5
Similarly, it is forbidden to mix a few bad fruits with many good fruits in to order to sell them under the presumption of [them all being] good, or to mix some bad drink with good.
However, if [the few bad fruits’ or the bad drink] taste was obvious it’s allowed to mix, because the buyer will notice.
The first paragraph is the same as the previous law, that one can’t doctor merchandise to sell it under false pretenses. This last clause is clarified in the second paragraph — that it’s okay to sell a batch or mixture of uneven quality as long as the buyer will certainly know that the purchase does include inferior product. That isn’t false pretenses.
However, as per the law we saw in 62:1, one may not overcharge. The buyer knowing the product is of mixed quality, but selling it to him at the market price for top quality product, could be ona’ah. Unless there is so little of the low-grade product that the market prices are nearly the same anyway. Otherwise, he would have to lower his price to the weighted average of the two quality lines sold.