I'm kind of with DreaminGirl on this.

It seems you're getting to a point with this character set that your time is being wasted trying to improve it when you could be spending that time on a new character set that might sell better. Spend some time looking at the character sets that are selling the highest, and try to go with something similar, if you want to try making real money. The pet projects are for spare time when you want to make a character because you want to make it, regardless of how well it sells. I think that's the case with the Tina character. She's not very attractive to a wide enough audience. She has a certain level of cuteness, but the mouth is where it falls short. Plus she's extremely young looking, which might appeal to some people, but could put off a lot of others for being "too young". If you're basing her on someone you know in real life, then your personal opinion of her is going to find her more attractive than the average stranger, because you know that girl's personality and that adds to her physical appeal on a mental level. It's hard to convey that, especially in a 3D model, to strangers. All the marketing and promo advice has it's merits, but it doesn't matter how great the promos are, most people are going to look at the final images of the character and ignore the descriptions.

I remember reading an article about a designer who made ceramic dinnerware. She hated it, because it wasn't what she wanted to do as an artist, but it's what paid her bills. In her free time she made the art she wanted to make and sold it on the side, but it was never enough to pay her bills. And that's pretty much what most artists have to deal with. You make the art that sells to make the money, and make what you want to make on the side, for extra money or just because that's what you want to do.

You might try attempting this character again at a later point after you've produced a few more that are more mainstream, and see if you can take a different approach with her. Sometimes it's best to step away from a project for a while and come back to it after you haven't seen it in a while with a fresh set of eyes. That tends to show you a lot of things you didn't notice before.

It also takes a while to build up a reputation, especially in this market, and with that reputation comes higher sales. There's a lot of competition, so it can be difficult to find a niche that you're good at that also produces high volume sales. Steer away from using merchant resources as much as possible, and learn to create your own textures from scratch. That will go a long way with helping to increase sales. If customers see the same skin texture on your characters that they see on 20 other characters, they're less likely to buy it unless you include something unique that the other 20 don't include, such as a new outfit, or some kind of accessories. That of course requires learning how to build and rig clothing if you don't already know how.