I have always liked old rosaries and have over the years bought many rosaries at various places. I have never re-sold any rosary I have bought, but the question of simony has come up in my conscience lately. Is it sinful for me to buy these, knowing that many of the old ones are most likely blessed?
The Catechism defines simony as the buying or selling of spiritual things (cf. CCC 2121). By your account, it is not your intention to buy a spiritual good (such as the graces using the rosary with the proper disposition can help us achieve) but only the rosary itself. This is not a sin. When you are ready to use them, you should have them blessed again.