You have to figure out your basis and accumulated depreciation. Taxes on sale of the property involve depreciation recapture tax and capital gain tax. Depreciation to be used is the allowed or allowable, which means if you didn't depreciate as you were required to do, you still have to pay depreciation recapture tax anyway as if you did. There are exceptions, and not a whole lot more can be said without further information.
The "Tax Basis" is the fulcrum of this transactiion. Tax Basis is calculated typically as follows:
Purchase Price + Improvement Costs - (Accumulated Depreciation) (Depreciation may or may not apply) = Tax Basis. The Tax Basis is then compared to the Sales proceeds ($50,000) to calculate gain or loss. If the Sales proceeds exceed Tax Basis the difference is the amount of the gain. In reverse and you have a loss.