I'm not too familiar, however having higher Herblore/Summoning/Dung always increases the price of skillers. Those seem to be the most sough after 99's. Cosmetic ones like agility/thieving etc are also good options to have.
Easy to get 99's that can be completed within less than a week e.g. Firemaking, cooking, etc... tend to go for around 15-35m. However accounts with 99's that take longer dung, herblore, slayer, etc... go from around 100-300m however having a good number of vouches always helps increase the price an account sells for as well.