Segment Information - Schedule of Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total revenue	$ 70,328	$ 69,666	$ 61,786	$ 55,751	$ 55,890	$ 47,722	$ 52,824	$ 49,504	$ 257,531	$ 205,940	$ 226,525
Gross profit	$ 40,061	$ 41,213	$ 35,401	$ 33,178	$ 33,163	$ 25,228	$ 26,205	$ 25,328	149,853	109,924	113,280
Operating Segments | GovDeals
Segment Reporting Information [Line Items]
Total revenue									49,579	32,806	32,936
Gross profit									47,030	30,721	30,386
Operating Segments | RSCG
Segment Reporting Information [Line Items]
Total revenue									158,806	136,491	127,321
Gross profit									64,564	49,727	44,967
Operating Segments | CAG
Segment Reporting Information [Line Items]
Total revenue									39,645	29,481	60,242
Gross profit									29,324	22,714	32,679
Operating Segments | Machinio
Segment Reporting Information [Line Items]
Total revenue									9,559	7,213	5,598
Gross profit									8,992	6,813	5,196
Corporate & Other
Segment Reporting Information [Line Items]
Total revenue									(57)	(51)	428
Gross profit									(57)	(51)	52
Revenue
Segment Reporting Information [Line Items]
Total revenue									146,151	127,580	147,889
Revenue | Operating Segments | GovDeals
Segment Reporting Information [Line Items]
Total revenue									0	0	0
Revenue | Operating Segments | RSCG
Segment Reporting Information [Line Items]
Total revenue									130,790	118,398	110,736
Revenue | Operating Segments | CAG
Segment Reporting Information [Line Items]
Total revenue									15,361	9,182	36,684
Revenue | Operating Segments | Machinio
Segment Reporting Information [Line Items]
Total revenue									0	0	0
Revenue | Corporate & Other
Segment Reporting Information [Line Items]
Total revenue									0	0	469
Fee revenue
Segment Reporting Information [Line Items]
Total revenue									111,380	78,360	78,636
Fee revenue | Operating Segments | GovDeals
Segment Reporting Information [Line Items]
Total revenue									49,579	32,806	32,936
Fee revenue | Operating Segments | RSCG
Segment Reporting Information [Line Items]
Total revenue									28,016	18,093	16,585
Fee revenue | Operating Segments | CAG
Segment Reporting Information [Line Items]
Total revenue									24,284	20,299	23,558
Fee revenue | Operating Segments | Machinio
Segment Reporting Information [Line Items]
Total revenue									9,559	7,213	5,598
Fee revenue | Corporate & Other
Segment Reporting Information [Line Items]
Total revenue									$ (57)	$ (51)	$ (41)