Many people invest so much of money on steemmonsters game by buying some good cards to upgrade their deck. There are also people who sell their existing useless cards for a good price and buy some cards that are useful for them. People who get good high-value cards from the quest rewards and season rewards go for such trading to buy enough summoners and level up. This is when there can be instances where you might have more neutral monsters than you actually require.
In my opinion, neural monsters are also precious like other rare, epic and legendary cards. There will not be high availability for neural monsters and that is the reason why the price of neutral monsters is always appreciating. If you wish to sell some of your existing cards to get other useful monsters, I would suggest having neutral monsters as the last option because the price we see in the market today may not be the case after few months. It keeps appreciating.
But even in that case, there are few neutral monsters available in abundance. For example, Highland archer is one such card which almost everyone would have. Such cards may not have any great value in the market and at the same time, we will not be able to guarantee the increase in the price of those cards.
Disclaimer: I'm just sharing my opinion here and please don't consider this as fail-proof financial advice. Do your own research when it comes to buying and selling.