As far as I know, simony refers to the practice of selling Church offices. I don’t think it has anything to do with blessed objects.
When it comes to transactions dealing with blessed items, in the circumstances you described, if there is a sin at all, it would be incurred on the part of the seller, not the buyer. If there is a statue of the Blessed Virgin Mary at a Goodwill, all the better that you purchase it and put it to good and reverent use. In a sense, you are rescuing the item from the very real possibility of profanation. Who knows, it could have fallen into the hands of someone with less benign intentions had you not bought it.
I know of people who have even purchased relics off of sites like eBay to save them from profanation. The selling of relics, of course, is strictly forbidden by Church law. However, when such holy items, by whatever means, fall into the wrong hands, it can be a salutary thing to purchase them back so as to venerate them and show them the dignity the deserve. Once again, in such a situation, the one who is selling the relic is committing the sin. The one who is buying it, if he or she has holy intentions, actually performs a good and reverent deed by doing so.