Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows From Operating Activities
Net income	$ 3,342	$ 2,194	$ 1,885
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	358	331	244
Stock-based compensation	1,486	1,415	1,364
Amortization of premium (discount) on available-for-sale investments	141
Write-off of deferred costs and long lived assets	3		57
Inventory valuation adjustment	164	710	301
Deferred income taxes	78	(117)	92
Noncash lease expense	684	643
Changes in operating assets and liabilities:
Accounts receivable	(11)	129	(143)
Inventory	(3,017)	(4,551)	339
Prepaid expenses and other current assets	330	128	42
Other assets	(13)	(11)	(56)
Accounts payable	(76)	621	151
Accrued expenses	546	1,231	145
Customer deposits	(636)	1,369	(4)
Lease liabilities	(101)	(350)
Other liabilities	(553)	(325)	23
Net cash provided by operating activities	2,725	3,417	4,440
Cash Flows From Investing Activities
Purchase of property and equipment	(508)	(316)	(376)
Purchase of debt securities	(12,594)	(17,737)
Proceeds from maturities of debt securities	11,009
Net cash used in investing activities	(2,093)	(18,053)	(376)
Cash Flows From Financing Activities
Net proceeds from exercise of stock options	176	174	407
Payment for shares withheld for tax withholdings on vesting of restricted stock units	(115)	(88)	(9)
Payments of deferred costs			(57)
Net cash provided by financing activities	61	86	341
Net increase (decrease) in cash and cash equivalents	693	(14,550)	4,405
Cash and cash equivalents at beginning of period	11,391	25,941	21,536
Cash and cash equivalents at end of period	12,084	11,391	25,941
Supplemental Disclosure of Cash Flow Information
Cash paid for income taxes	67	35	44
Supplemental Disclosure of Noncash Investing and Financing Information
Property and equipment purchased but not yet paid	51	89	173
Vesting of early exercised options	$ 44	$ 67	$ 74