The Company has three stock option plans that provide for the award of stock options to selected employees, directors, and officers of the Company: a) Amended and Restated Stock Option Plan, b) IronPlanet 1999 Stock Plan, and c) IronPlanet 2015 Stock Plan.

Stock option activity for the six months ended June 30, 2021 is presented below:

The significant assumptions used to estimate the fair value of stock options granted during the six months ended June 30, 2021 and 2020 are presented in the following table on a weighted average basis:

As at June 30, 2021, the unrecognized stock-based compensation cost related to the non-vested stock options was $8,979,000, which is expected to be recognized over a weighted average period of 2.4 years.

The total market value of liability-classified share units vested and released during the first half of 2021 was nil (December 31, 2020: nil).

Senior executive and employee PSU plans

The Company grants PSUs under a senior executive PSU plan and an employee PSU plan (the “PSU Plans”). Under the PSU Plans, the number of PSUs that vest is conditional upon specified market, service, and/or performance vesting conditions being met. The PSU Plans allow the Company to choose whether to settle the awards in cash or in shares. The Company intends to settle by issuance of shares. With respect to settling in shares, the Company has the option to either (i) arrange for the purchase shares on the open market on the employee’s behalf based on the cash value that otherwise would be delivered, or (ii) to issue a number of shares equal to the number of units that vest.

Fair values of PSUs are estimated on grant date using the 20-day volume weighted average price of the Company's common shares listed on the New York Stock Exchange.

As at June 30, 2021 the unrecognized share unit expense related to equity-classified PSUs was $14,431,000, which is expected to be recognized over a weighted average period of 2.1 years.

RSUs

The Company has RSU plans that are equity-settled and not subject to market vesting conditions.

Fair values of RSUs are estimated on grant date using the 20-day volume weighted average price of the Company's common shares listed on the New York Stock Exchange.

As at June 30, 2021, the unrecognized share unit expense related to equity-classified RSUs was $2,323,000, which is expected to be recognized over a weighted average period of 1.6 years.

19. Share-based payments (continued)

DSUs

The Company has DSU plans that are cash-settled and not subject to market vesting conditions.

Fair values of DSUs are estimated on grant date and at each reporting date. DSUs are granted under the DSU plan to members of the Board of Directors. There is no unrecognized share unit expense related to liability-classified DSUs as they vest immediately and are expensed upon grant.

As at June 30, 2021, the Company had a total share unit liability of $8,699,000 (December 31, 2020: $9,597,000) in respect of share units under the DSU plans.

Employee share purchase plan

The Company has an employee share purchase plan that allows all employees that have completed two months of service to contribute funds to purchase common shares at the current market value at the time of share purchase. Employees may contribute up to 4% of their salary. The Company will match between 50% and 100% of the employee’s contributions, depending on the employee’s length of service with the Company.