Not sure why you're being downvoted, that's a legitimate question. The ATF spends a lot of time going after illegal silencers (usually sold as 'oil filters') but they generally focus on end users who buy in quantity or the manufacturers who 3d print them. Have not heard of them going after marketplaces like eBay, but then again marketplaces generally bend over backwards to maintain compliance with firearm regs because they don't want to see a headline like 'mass shooter bought weapons on ebay'.