1) Can you buy from a store that may be selling items that were stolen or is it lifnei iver?
Answer:
If you know that the items in question are stolen, it would not be permitted to buy them.
This is not only a prohibition of lifnei iver. It is forbidden to buy stolen property, which involves a further removal of the property from its owner. If one buys stolen property from a known thief, of property that one knows to be stolen, there would be an obligation to return the item to its owner.
There would also be an issue of “mesayai’a li-dvar aveirah,” abetting somebody in his wrongful practice, and generally strengthening the hands of thieves.
In general, however, one would only need to be concerned for this where there is concrete grounds for suspecting that the property is stolen. See Bava Kama 119a (and Rashi), where we find that for goods sold in the public domain, we can assume that the goods are not stolen, for otherwise the seller would try to hide them.
