Here are a couple examples of my perspective of this topic:
A1 wants item X on account A2, their alt
B has item X as well, and is willing to help A1, but wants to keep item X
B gives item X to player A1 and gets a new item X from A2
This is abuse. Even though at no point does the item from A2 go to A1, B would never have given the item if A2 was not going to replace it. Fundamentally, it should be considered as the same item.
A1 wants item X1 on account A2, their alt
B has item X1 as well, and wants items X2 from player A1, but also wants to keep X1
B trades item X1 to A1 in exchange for item X2 from A1, and X1 from A2
This is also abuse. Again, at no point does the item on A2 go to A1, and A1 has even payed B for item X1, but A2 paying part of the cost is still resulting in an abuse - you can't use items from one of your accounts to buy items for another.
A1 wants item X1 on account A2, their alt
B has item X1 as well, and wants items X2 from player A1
B trades item X1 to A1 in exchange for item X2 from A1
A2 retains their copy of item X1
This is not abuse. In this case, the only exchange is a proper trade, with no alts involved. A2 might not need item X1, but no effort to move the item to A1 has been made - a completely discreet copy has been acquired.
A1 wants item X1 on account A2, their alt
B has item X1 as well, and wants items X2 from player A1, but also wants to keep item X1
A2 can make use of item X3, which B has and is willing to trade
B trades item X1 to A1 in exchange for item X2 from A1
A2 trades their copy of X1 for item X3 from B
This is not abuse. While B is ending up with a copy of X1 at the end, this is not as a result of a single trade, but two discreet trades. This is a touch messier, because the existence of the second trade is a deciding factor in whether or not the first trade happens. However, the second trade is a discreet trade, with all goods payed for as part of the trade. It is not unknown for someone to make a trade with one person, selling something they want, because they know someone else will pay them more than the price they payed for the same item. This is, in essence, the same as a supermarket making a markup in price.
The other scenario's are fairly clear cut, but this last one is the one that requires monitoring - paying a significant value item, such as a Morph, on the second trade would very rarely be accused of abuse, while paying a low value, such as a flower or single silver coin, would almost always be abuse.