[TeMPOraL]

Indeed, this follows from "efficient market hypothesis", "TANSTAAFL", etc. and probably has some more solid theoretical grounding too: if you're getting a good bargain, someone is getting screwed. Quite likely it's you.

It may not seem like that to the bargain hunters, because they're being used indirectly (the bargain is there to get them into the store), longer-term (the bargain is there to form a habit), or stochastically (the bargain may screw over only subset of buyers, so while on average, the buyer is losing, you in particular may be winning this time around). Sometimes the scope is even larger (e.g. the vendor burning money to dig themselves into the market). Or, it may be just the buyer exploiting some corner case so obscure it's not visible unless your time is literally free.

It's a simple idea that's hard to swallow: the bargain benefits the dealer somehow, or else it wouldn't be offered.

[teaearlgraycold]

For one off events you can have genuine bargains. I give away perfectly good computer hardware for free on Craigslist or at steep discounts.

[Guvante]

You don't need anyone to get screwed.

That only applies if finding a buyer is free which it objectively is not.

Sometimes the effort to offload something for its value is more than that value.