it´s openly played racism
Since no race was ever mentioned, it's not.
Racism should not be the first thing people jump to the second something inconveniences someone. This works the same as the Brady bunch's claims of gun owners snapping--it's what, around here, we call 'projection'.
Aside from that, everyone involved was in the wrong.
The buyers for asking about converting it, discussing who best would pass the NICS, and general stupidity.
The seller for selling it after them asking, discussing, and then setting them up.
If I took my AK to the show, and someone asked "How easy would that be to convert to FA", they will get a polite but very firm "no idea, it's very much illegal, and I'll have no part in it." From that point, I will answer their questions about that if they just didn't know or, if they persist, tell them to have a good day, walk away, and alert security. I'll either postpone a crime waiting to happen, or get an ATF 'informant' (ahem) or Bloomberg thug kicked out, and either is just dandy by me.
If I was a dealer, after the way these guys acted, I would simply tell them "I don't feel comfortable selling to you. I'm suspicious you will illegally manufacture a machine gun/commit a straw sale/intend to carry illegally/etc. You can find the pamphlet on local laws over at that booth. Have a nice day." and ask security to keep an eye on them.
Even if they passed the NICS,
talking over which one would be most likely to means they either didn't expect to pass it up, or it's a straight-up straw sale. No thank you, sir.