Is there any particular reason, other than “because Blizzard said so,” that we can’t mail our own toons on the opposite faction (besides BOA’s)? Just wanted to send a few Hexweave Bags and little bit of gold to get them started off, but nope apparently my own toon is unfriendly  . Do you think it should be allowed, or is there actually a valid reason that it isn’t allowed?
. Do you think it should be allowed, or is there actually a valid reason that it isn’t allowed?
You already answered your own question.
You could just ask a friend to buy something from the neutral auction house for how ever much gold you want to transfer then give them that gold from your main
It’s not the easiest solution but it’s a solution
Ignore comment I lacked some information
The AH have been merged for a few expansions.
Ok I don’t play much in the AH so I didn’t know my bad.
All good. 
Unfortunately, I only have one friend who plays this game and they only play the Horde so I don’t think that’ll work. Unless I’m misunderstanding what you’re saying.
Edit: Actually I’m sure he’d make an Alliance character for this its not like that its too much of a hassle lol. Not like that character needs to be max level or anything.
If you have any duplicate battle pets, you can cage them on your Alliance character and sell them in the AH.
You answered your own question
I don’t see a problem with mailing alts on the opposite faction. However, I don’t make the rules.
You can get a few battle pets that sell on your realm then cage them on your alts to sell on the auction house. That is what I have done in the past.
There is also the old fashioned Auction House transfer as well. With your horde character put a junk item like a feather on the auction house for a high value. Have your friend create an Alliance character. Give that character the gold to buy the junk item. Collect your gold in an hour.
Only have one friend that plays, and he only plays on the Horde, but I’m sure he wouldn’t mind making an Alliance character for this so I’ll give that a try. I’ll also try the pet idea as well.
I’ve helped peeps transfer items back and forth from time to time. As far as bags go, I would just put pets in cages, as previously stated, and just buy bags with the gold.
On a side note, Many crafted BFA items don’t even show up in auction for opposite faction, so I wish they would show up as equivalent items on other side since prices are out of wack on servers that aren’t balanced.
We need a server wide, cross faction storage system and the ability to mail anything cross faction on the same server. It’s pretty dumb that we can’t.
If you are DESPERATE, buy a bunch of BOA Epic guild tabards and send  to your alt. Sell them. Take a gold loss. But hey, your alt has gold now 
No a horde character will work. Have your alliance toon put a trash item for what ever gold you need. Your friend buys that item and now your alliance toon has the gold. Then you pay your horde friend back the amount the Alliance guys auction was for
This might work in the regular auction house too because they are faction linked
Why does he need a friend? Can’t he just buy the item himself?
You cannot buy your own auctions. Even if they are cross-faction.
Ahh okay. I had no idea. I don’t really use the AH much.
You can use a second account but since trial accounts can’t carry any worthwhile amount of gold and cannot trade, it would require a month’s subscription. A friend helping out for 10 minutes is cheaper.