EDITED: I talked to a Mod on December 11, 2016 to clarify.
Also on Dec. 12,2016.
Off-site trading to and from Horse Isle 2 is allowed, just not recommended, as I don't think off-site trading on any site is exactly recommended, Horse Isle 2 does not ban it. I have talked to an admin already. It is also stated in the rules under the letter "T" and then under"Trading Between Games" and "Trading On a Promise"
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"Trading Between Games
1] Trading between the Horse Isle games (Secret Land of Horses/HI1 and Legend of the Esrohs/HI2) is not recommended or endorsed by Horse Isle.
2] Trading between the games includes, but is not limited to, getting money on one game for a specified amount of money on the other game.
3] This is considered trading on a promise and not a good idea. Should you decide to do this anyway, realize Horse Isle will not return any stolen funds to you should someone not complete their end of the deal.
4] If a player does agree to trade with you and they do not follow through, please do file an abuse report, but again, you will not get your money, items or horses back.
5] Because this is trading on a promise and not advised, advertising is not allowed.
Trading on a Promise
1] Trading on a promise is when one player gives something to another under the promise that the other will do their half of the trade, or return the traded item, at a later time.
2] Horse Isle strongly advises to never do a trade on a promise. Always be sure a trade is fair in and of itself.
3] Trading on a promise includes, but is not limited to: trading between HI1 and HI2, boarding a horse with another player not using the leasing system, lending items/pets/money, trading an item/horse/money with the player promising to pay you back later, having a horse trained without using the in-game system.
4] If you do complete a trade on a promise and the other player does not fulfil their end of the deal, please file an Abuse Report immediately.
5] Horse Isle does not return items/horses/pets/money that have been stolen. So be very sure you are happy with a trade before you accept it."
Here: http://hi2.horseisle.com/www/detailedrules.php