That's a lie they're feeding you and you've fallen for it, hook, line, and sinker. There's no online loophole. Online sales must be shipped to an FFL and a background check passed before delivery - with the single exception that makes it a private sale where, for instance, I advertise my gun on a site but we meet face-to-face to sell it so it's a private sale, not an online sale.
Most gunshow sales require a background check. A dealer cannot sell a gun at a gunshow without the same paperwork and process that they would do at their brick and mortar store front. If a private person, not a dealer, goes to a gun show and sells you a gun, just like if they met in his living room or the walmart parking lot, it's a private sale, not a gunshow sale. Gunshows are extremely careful about monitoring sellers to make sure the laws are complied with. If a private seller rents a table at multiple gunshows to the point that it's questionable whether they're just selling their collection or if they're actually in the business of selling, then the gunshow operator will ban them in a heartbeat.
I'm getting on in years and have a moderately-sized gun collection. I've been considering selling my collection. If I sell what I have today, at a gun show or in the Walmart parking lot or in my living room, those are private sales - no background check required. It's not a gunshow loophole.
I have one son-in-law that I probably wouldn't give a tourniquet to if he cut his leg off but he has one of my grandsons. I would consider transferring my collection to my grandson, and I might, but son-in-law and grandson are avid hunter type gunowners. They fire their rifles three times each fall to sight in their scopes, two more times each to get their limit on deer, and then put them away until the next season. Though they really do think they're protectors of the 2nd Amendment, they understand the real intent and meaning of the Amendment no more than most people here so I can't count on them to take seriously the responsibility of owning and defending the right to own black modern sporting rifles.