Cost of sales in TechOps decreased $7.3 million, or 9.5%, to $69.5 million for the nine months ended September 30, 2023, compared to the prior year period, driven by lower costs related to the sale of Flight Equipment of $16.1 million during the nine months ended September 30, 2022, partially offset by cost associated with revenue fluctuations noted above. Gross profit in TechOps decreased $6.9 million, or 25.6% for the nine months ended September 30, 2023, compared to the nine months ended September 30, 2022, driven by lower profit generated from the sale of Flight Equipment of $7.1 million and USM sales, partially offset by higher gross profit of $0.8 million on MRO services. Gross profit margin decreased to 22.4% for the nine months ended September 30, 2023 compared to 26.0% for the nine months ended September 30, 2022, and was largely attributable to the margin generated from the sale of Flight Equipment of 31.9% for the nine months ended September 30, 2022, as well as lower margins on MRO services of 21.7% for the nine months ended September 30, 2023, compared to 22.5% during the nine months ended September 30, 2022.
Selling, General and Administrative Expenses
Selling, general and administrative expenses increased $6.5 million, or 9.1% to $77.7 million for the nine months ended September 30, 2023, compared to the prior year period. The increase was mostly related to Company-wide cost of living adjustments, additional headcount, along with higher facility and legal costs incurred.
Change in Fair Value of Warrant Liability
We account for our private warrants as a liability at their fair value, with changes in fair value recognized in our results from operations for the period. The fair value of our private warrants is determined using a Black Scholes option pricing model. For the nine months ended September 30, 2023, we recorded a change in fair value of the warrant liability gain of $1.0 million, compared to a $1.9 million loss in the prior year period.
Interest Income (Expense), Net
Interest income, net for the nine months ended September 30, 2023 was $1.2 million and was primarily related to interest generated on excess cash. There was nominal interest expense during the nine months ended September 30, 2022.
Income Taxes
The effective tax rate for the nine months ended September 30, 2023 was 59.8% compared to 22.2% for the nine months ended September 30, 2022. The difference between the effective tax rate and the statutory tax rate of 21% for the nine months ended September 30, 2023 is primarily due to the impact of state income taxes and non-deductible executive compensation, offset by the foreign derived intangible income deduction, release of the valuation allowance and R&D credits. The difference between the effective tax rate and the statutory tax rate of 21% for the nine months ended September 30, 2022 is primarily due to the impact of state income taxes and non-deductible executive compensation, offset by the foreign derived intangible income deduction.
Financial Position, Liquidity and Capital Resources
As of September 30, 2023, we had $3.2 million of cash and cash equivalents. We finance our growth through cash flows generated from operations and borrowings secured by our assets. We had $8.6 million outstanding under the Revolving Credit Agreement as of September 30, 2023, and we had $149.0 million of availability thereunder. We used cash in operations of $168.1 million for the nine months ended September 30, 2023, mostly for feedstock acquisitions, and generated cash from investing activities of $6.7 million for the nine months ended September 30, 2023.
During the nine months ended September 30, 2023, we entered into a revolving term loan collateralized by our property and equipment (the “Equipment Loan”), and borrowed $8.6 million, which remained outstanding as of September 30, 2023.
We believe our equity base, internally generated funds, and existing availability under our debt facilities are sufficient to maintain our level of operations through September 30, 2024. If an event occurs that would affect our ability