There's a lot of factors that can change the price of a comic book..
Yes, in most cases the entire set would sell for a lot more than just one or two from the set. But there could be someone out there that just needs that 'one' comic to complete their collection. They might be willing to pay more for it because of the value it has for them individually. It could just be a matter of finding that person who will truly enjoy having them and pay a lot to get them.
There are also a lot of people that would rather the comic be graded for resale purposes, or just for the sake of knowing that a professional agrees with what you say it is.
It's tricky, but you have to think about everything when figuring out what to do with them..
Also, i think it would be best to send in one each of your TMNT comics, and definitely the Death of Superman. And save that, because it will go for A LOT.
Also, did you upload your inherited collection onto Nosto? That can help others see what you have and possibly help with pricing and worth.