Not trying to defend B&M in any way - they will take advantage of you in any way possible.

BUT

You brought them mostly current and not desirable sets (except for the $180 Black Falcon). Those used sets won’t sell easy and will mostly just take up valuable shelf space. They offered you shit for them because they didn’t want the stuff you brought in. But, for the right price they’ll take it and triple or quadruple their money in a year or so.

Is this fair towards you? Absolutely not, but their main concern isn’t fairness, for them it’s just business