Can I sell them online without violating trademark laws?

While it is unclear what them is, given your question I will assume they are consumer or other goods protected by a trademark. You should be very careful about reselling items for which you have no authorization especially if they are covered by a famous brand. It is true that Customs either destroys counterfeit items that are seized at the ports or airports or offers them to the brand owner, but that should not have any bearing on the goods you plan to resell. It is possible that they are gray market goods, legitimate product that found its way into the US market from another legitimate distributor. But again, without permission of the authorized distributor in the US or your region, you could wind up with a very upset distributor who may come after you. The only legitimate resale of product is from a consumer who purchased the product at retail and is reselling it or if there is a legitimate overstock sale from a legitimate liquidator. Bottom line make sure the source from which you obtained the product is legitimate, and if possible, get assurances in the form of indemnification should it turn out the product you resell came from an unauthorized source. Ignorance will not be an excuse or shield from liability including potentially criminal liability if the product you are reselling turns out to be counterfeit.