Trade definitely played a very important role in the kingdom of Axum. Whilst agriculture and cattle were also vital to developing Axum's strength and power, it was mainly trade that helped Axum to its success.
One explanation that you might want to point out for this is the fact that Axum controlled some important trade routes due to its geographical position. This allowed them to benefit significantly from trade, as Axum was able to buy and sell various goods. In fact, you could even point out that Axum was the first sub-Saharan African state to develop its own coinage. You could use this as evidence for the importance of trade. After all, coins and money are important for trade, so the development of its own coinage clearly shows how important trade was for the kingdom of Axum.
In terms of selling goods, Axum very much benefited from its stock of gold and ivory. For example, Axum did a lot of gold and ivory trading with the Byzantine Empire. Other goods used for export and trade were salt, tortoise shell, and incense. You could also point out that Axum was also in possession of gemstones, which is why they also used emeralds and obsidian to trade with. In return, Axum bought items such as weapons, textiles, glassware, and other luxury goods.