As a LEGO reseller, depending on what I bought, I might consider asking this but definitely in a far more laid back tone such as
Missed out on this set and now I got pay tons of money for it! Oh well, I make a profit like you also by selling LEGO at exorbitant prices. LOL. Thanks!
and then depending on what it is, I might follow with
Just curious, how long did this sit in your storage?
Depending on the tone you read it in, the message could have different connotations. I can clearly see how you think it could be a scam, but it it was meant as a joke they probably should have added a ":)" or a "lol" to convey that they aren't angry or upset with you the seller.