I don't know how it is for US customers, however I can't order anything to be sent to Greece from UK tobacco shops.
I believe it has to do with EU laws: since 2016 the EU allows countries to decide whether they allow internet sales or not. I don't know if this applies only to cross-border sales but I do know that if I look into UK websites from an EU IP then across the board all websites state "no shipping outside the UK". It may be different from a US IP. The Danish pipe shop won't ship to me either, however Estervals or hacico (German) will.
Similarly there is a restriction to what amounts/packaging can be sold within the EU: if I look into Estervals or Synjeco I get only 50g, or 5x50g options, no bulk, this I am sure it has to do with my IP again as I'm sure I've read here that US people can buy by the kg.
I believe each country sets their own restrictions, for example in Greece tobacco can only be legally sold in a sealed container with a specific sticker across the seal stating that tax has been paid, which means no loose jars, no making your own mix etc.
Regarding buying from a person, no clue!