Here you go, Mr. Pig.
1. There has been no established rule for purchasing open market items from a realm you are not currently in. So, you could go off and buy a Zeus from the Lyran Alliance while sitting on Luthien. However, traditionally, the process has always been that for open market items, you buy from the nation you are in or from Terra and have the items shipped to you. Still there is no rule that says you couldn't buy from the LA while in DC space but you may have the ISF watching you more.
2. Yes. Assuming that you are purchasing from the realm you are in, the item ships to you from the nearest depot.
3. Here are the charges for shipping
•1.5% Chance in the same Realm Charge: tonnage * 60 - Insurance fee 5% (Optional)
•2.5% Chance across Realms Charge: tonnage * 60 plus a 7,500 C-bill Customs fee Insurance fee 6.25% (Optional)
•3.5% Shipped to a Unit in or beyond Periphery Insurance fee 7.5% (Optional)
Additional information on MHB purchasing can be found on the MHB page under "Basics". Here's the link
https://www.battletech-mercenaries.com/mhb/basics.phpAt this time, I am also the MHB person, so if you have any additional questions please feel free to ask me.