Market sell order have minimum size although there was announcement that minimum order sizes will be removed - https://www.coinbase.com/blog/upcoming-changes-to-order-size-limits.
From blog post:
We will maintain the quote min order size limit as the check on notional. The check will apply to all orders where funds in the quote currency are specified (e.g. limit orders and market buys).
Market sell is specified in base currency. Trying to sell 1 SHIB I am getting following error:
{"success":false,"failure_reason":"UNKNOWN_FAILURE_REASON","order_id":"","error_response":{"error":"UNKNOWN_FAILURE_REASON","message":"","error_details":"","preview_failure_reason":"PREVIEW_INVALID_BASE_SIZE_TOO_SMALL"},"order_configuration":null}
I can sell 1 SHIB from Coinbase Pro web interface and/or from API.