Segment Information - Schedule of Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2026	Mar. 31, 2025
Segment Reporting Information [Line Items]
Total revenue	$ 120,732	$ 116,375	$ 241,952	$ 238,706
Cost of goods sold (excludes depreciation and amortization)	64,862	68,946	129,048	141,110
Segment direct profit	55,870	47,429	112,904	97,596
Operating Segments
Segment Reporting Information [Line Items]
Segment direct profit	55,870	47,446	112,904	97,630
Operating Segments \| GovDeals
Segment Reporting Information [Line Items]
Total revenue	21,291	19,236	43,557	39,758
Cost of goods sold (excludes depreciation and amortization)	1,410	1,524	2,499	3,230
Segment direct profit	19,881	17,712	41,058	36,528
Operating Segments \| RSCG
Segment Reporting Information [Line Items]
Total revenue	83,225	82,692	165,220	170,373
Cost of goods sold (excludes depreciation and amortization)	61,860	66,123	122,394	135,309
Segment direct profit	21,365	16,569	42,826	35,064
Operating Segments \| CAG
Segment Reporting Information [Line Items]
Total revenue	10,776	9,592	22,260	19,443
Cost of goods sold (excludes depreciation and amortization)	1,122	940	3,227	1,995
Segment direct profit	9,654	8,652	19,033	17,448
Operating Segments \| Machinio & Software Solutions
Segment Reporting Information [Line Items]
Total revenue	5,440	4,872	10,915	9,166
Cost of goods sold (excludes depreciation and amortization)	470	359	928	576
Segment direct profit	4,970	4,513	9,987	8,590
Corporate expenses, including elimination adjustments
Segment Reporting Information [Line Items]
Segment direct profit	0	(17)	0	(34)
Purchase revenues
Segment Reporting Information [Line Items]
Total revenue	77,852	77,827	155,204	160,642
Purchase revenues \| Operating Segments \| GovDeals
Segment Reporting Information [Line Items]
Total revenue	1	51	1	563
Purchase revenues \| Operating Segments \| RSCG
Segment Reporting Information [Line Items]
Total revenue	75,965	76,451	149,955	157,136
Purchase revenues \| Operating Segments \| CAG
Segment Reporting Information [Line Items]
Total revenue	1,886	1,325	5,248	2,943
Purchase revenues \| Operating Segments \| Machinio & Software Solutions
Segment Reporting Information [Line Items]
Total revenue	0	0	0	0
Consignment and other fee revenues
Segment Reporting Information [Line Items]
Total revenue	42,880	38,548	86,748	78,064
Consignment and other fee revenues \| Operating Segments \| GovDeals
Segment Reporting Information [Line Items]
Total revenue	21,290	19,185	43,556	39,195
Consignment and other fee revenues \| Operating Segments \| RSCG
Segment Reporting Information [Line Items]
Total revenue	7,260	6,241	15,265	13,237
Consignment and other fee revenues \| Operating Segments \| CAG
Segment Reporting Information [Line Items]
Total revenue	8,890	8,267	17,012	16,500
Consignment and other fee revenues \| Operating Segments \| Machinio & Software Solutions
Segment Reporting Information [Line Items]
Total revenue	$ 5,440	$ 4,872	$ 10,915	$ 9,166