Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (50,097)	$ (31,701)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	5,993	4,136
Stock-based compensation expense	2,397	1,226
Change in fair value of convertible note derivative liability		1,248
Bad debt expense	681	333
Compensation expense related to stock sales by current and former employees	819
Change in fair value of convertible preferred stock warrant liability	2,100	183
Accrued interest on convertible notes		223
Accretion of unconditional endowment grant liability	44	53
Accretion of debt discounts	9	97
Amortization of premiums on short-term investments	42	15
Changes in operating assets and liabilities:
Accounts receivable	(2,627)	918
Inventory, net	(2,309)	(1,453)
Prepaid expenses and other current assets	(867)	(3,913)
Other assets	411	70
Accounts payable	157	(1,480)
Accrued consignor payable	(1,855)	(3,872)
Other accrued and current liabilities	(1,744)	4,516
Other noncurrent liabilities	672	695
Net cash used in operating activities	(46,174)	(28,706)
Cash flow from investing activities:
Purchases of investments	(9,151)	(2,211)
Proceeds from maturities of short-term investments	22,898	7,600
Proceeds from sale of short-term investments		7,023
Capitalized proprietary software development costs	(3,887)	(2,245)
Purchases of property and equipment	(10,042)	(4,164)
Net cash (used in) provided by investing activities	(182)	6,003
Cash flow from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	43,492	86,640
Proceeds from issuance of convertible preferred stock, net of issuance costs	26,283
Proceeds from issuance convertible notes, net of issuance costs		14,273
Proceeds from exercise of stock options and common stock warrants	1,775	63
Payment of deferred offering costs	(3,057)
Issuance cost paid related to conversion of convertible notes		(545)
Repayment of debt	(2,750)	(1,500)
Net cash provided by financing activities	65,743	98,931
Net increase in cash, cash equivalents and restricted cash	19,387	76,228
Cash, cash equivalents, and restricted cash
Beginning of period	45,627	20,660
End of period	65,014	96,888
Supplemental disclosures of cash flow information
Cash paid for interest	219	324
Cash paid for income taxes	141	45
Supplemental disclosures of non-cash investing and financing activities
Issuance of convertible preferred stock upon extinguishment of convertible notes		10,557
Issuance of redeemable convertible preferred stock upon extinguishment of convertible notes		5,642
Accretion of redeemable convertible preferred stock to redemption value	3,355	2,451
Loss on extinguishment of convertible notes		370
Purchases of property and equipment included in accounts payable	(837)	$ (695)
Deferred offering costs in accounts payable and accrued liabilities	$ 2,219