depending on what it is, any of those retailers are pretty well known, final frontier had really good quality but the highest prices back when I was going hard on everything (but that was still about half of today's prices so who knows now)
I bought a damaged ESB card original Leia from Dallas Vintage a few years ago that was an incredible deal and absolutely pristine so I cant knock them too hard but maybe just be a little extra vigilant and shy away from anything questionable with them
Up to you, but it's worth considering that every dollar you put in his pocket is allowing him to stay in business and rip other collectors off. I can't condone the deliberate and harmful actions perpetrated by him so will never purchase from him.
These days there's rarely any reason to purchase vintage from a business like those guys. So many vintage items sold on Facebook these days through the various collecting groups.