Basis of Presentation and Summary of Significant Accounting Policies - Revenue Recognition (Details)	5 Months Ended	12 Months Ended	18 Months Ended
	Dec. 31, 2019 platform	Dec. 31, 2019 USD ($) platform	Dec. 31, 2019 platform	Jul. 15, 2018
Marketplace Revenue
Disaggregation of Revenue [Line Items]
Listing fee per item | $		$ 0.20
Period over which listing fee is recognized		4 months
Fee for each completed transaction, percent			5.00%	3.50%
Services
Disaggregation of Revenue [Line Items]
Number of unified ad platforms | platform	1	1	1
Minimum | Marketplace Revenue
Disaggregation of Revenue [Line Items]
Direct checkout fees, percent		3.00%
Maximum | Marketplace Revenue
Disaggregation of Revenue [Line Items]
Direct checkout fees, percent		4.50%
Reverb Holdings Inc. | Marketplace Revenue
Disaggregation of Revenue [Line Items]
Fee for each completed transaction, percent	3.50%
Reverb Holdings Inc. | Minimum | Marketplace Revenue
Disaggregation of Revenue [Line Items]
Direct checkout fees, percent		2.50%
Reverb Holdings Inc. | Maximum | Marketplace Revenue
Disaggregation of Revenue [Line Items]
Direct checkout fees, percent		2.70%