Segment Information - Reconciliation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total revenue	$ 55,751	$ 49,504
Gross profit	33,178	25,328
Operating expenses	28,548	30,125
Other operating expenses	4	193
Interest and other income, net	(185)	(252)
Income (loss) before provision for income taxes	4,811	(4,738)
Operating Segments | GovDeals
Segment Reporting Information [Line Items]
Total revenue	10,818	8,015
Gross profit	10,203	7,446
Operating Segments | RSCG
Segment Reporting Information [Line Items]
Total revenue	34,906	31,697
Gross profit	14,627	10,305
Operating Segments | CAG
Segment Reporting Information [Line Items]
Total revenue	7,913	7,942
Gross profit	6,358	5,814
Operating Segments | Machinio
Segment Reporting Information [Line Items]
Total revenue	2,114	1,850
Gross profit	1,991	1,763
Corporate & Other
Segment Reporting Information [Line Items]
Total revenue	0	0
Gross profit	0	0
Revenue
Segment Reporting Information [Line Items]
Total revenue	31,071	30,349
Revenue | Operating Segments | GovDeals
Segment Reporting Information [Line Items]
Total revenue	0	0
Revenue | Operating Segments | RSCG
Segment Reporting Information [Line Items]
Total revenue	28,161	27,826
Revenue | Operating Segments | CAG
Segment Reporting Information [Line Items]
Total revenue	2,910	2,523
Revenue | Operating Segments | Machinio
Segment Reporting Information [Line Items]
Total revenue	0	0
Revenue | Corporate & Other
Segment Reporting Information [Line Items]
Total revenue	0	0
Fee revenue
Segment Reporting Information [Line Items]
Total revenue	24,680	19,155
Fee revenue | Operating Segments | GovDeals
Segment Reporting Information [Line Items]
Total revenue	10,818	8,015
Fee revenue | Operating Segments | RSCG
Segment Reporting Information [Line Items]
Total revenue	6,745	3,871
Fee revenue | Operating Segments | CAG
Segment Reporting Information [Line Items]
Total revenue	5,003	5,419
Fee revenue | Operating Segments | Machinio
Segment Reporting Information [Line Items]
Total revenue	2,114	1,850
Fee revenue | Corporate & Other
Segment Reporting Information [Line Items]
Total revenue	$ 0	$ 0