This implies, though, that one has sufficient extra dollars to keep investing without funding new purchases from the sale of old ones. If instead one needs to redeem some in order to purchase others, the first thing to remember is that one hardly ever with this strategy needs to sell at a loss. A good company is a good company, and its price having gone down might be reason to buy more but almost never should be an occasion for selling. As Buffett suggests, if one had bought a nutritious loaf of bread, and the next day the loaf is on sale for 50% off, one would not throw out the first loaf, though one might buy a second one at this now bargain price.
If one finds he or she has a lot of stocks that after purchase routinely go down a lot compared with the market, it may not be reason to sell them, but could very well be cause for reviewing the buy strategy. Value investing, without special care for the profitability of one's purchases, can too often involve a lot of these "falling knife" kinds of assets, ones that are down for good reason and will thus keep going south. Maybe they will turnaround eventually, maybe not, but in the meantime one might have poor overall returns.