1. A bit confused and unclear on the "primary residence" part. Was it your primary residence for "at least" two of the last 5 years you owned it? If yes, then you will pay no taxes on any gain realized that is less than $250K.
2. Remaining mortgage has nothing to do with the selling price of your 50%. The best thing to do is to get an appraisal on the property and start your negotiations there. One thing that has to happen if your name is on the corrent mortgage, is that even if the other owner buys it, they will need to finance their purchase price to completely pay off the existing loan and take out a new mortgage that does not have your name on it. So long as your name remains on the existing mortgage, you have a legal obligation to pay it. It doesn't matter if you do a quit claim on the property deed either. That does not remove your name from the mortgage and does not release you from your legal obligation to pay it.
Overall, I would highly suggest you get with a realtor in your local area. While I would not expect a realtor to be informed so much on the tax side of things, they are familiar with the legalities of all this so that after the sale you can walk away "truly" free from any and all obligations on the property. A realtor also has a better "feel" for the market and can be more realistic in helping you negotiate a price. Generally the realtor gets up to 6% of your gross sales price as commission - it's how they make a living and it's generally worth it to both the seller and the buyer.
