Unhappy buyer .What to do ???
Pretty stand up of you to be this concerned
that's the whole thing as to whether the OP should reimburse the buyer (in the form of 'adjusting the price') or not.
If you feel bad about the deal (and, like the others have said, you really have no reason to) then you could send some money back (more for your own peace of mind than because you 'should').
If you're fine with what you sold them for, and how they were represented and sold (and, again, you should be) then tell the buyer that.
In other words, you have to do what makes you feel the best about the transaction, IMHO.