Agreed. Let the other trader know your thoughts (nicely go without saying...) Perhaps make a suggestion or two as to how you would like to resolve it (return trade, or pick out another from their traders), but also leave it open as to what they will offer. Do not be left with 'nothing' since the other trader did not disclose the issue.
I had an entire international trade go south when my trading partner's shipment was lost/never arrived at my address. I asked for my pins back..., but we settled on the other trader sending me another shipment with pins of about 1/2 the value of what I was to originally receive. Compromise...
Another suggestion for future trades... ask for pictures of the pin(s). I sometime forget to do this in my rush/excitement to trade, but as they say, "a picture is worth a thousand words".