A bird in the hand is worth 2 in the bush. If the offer was 10 and you would accept 11 then you are almost there. sell it for 10 without the sound system and ebay it.
When it comes to selling something, it's not nessesarily what "you think it's worth" but what it's worth to someone else. If you are in no hurry to sell and move on then stay on it. If your rego is about to come up then It's time to rethink your selling strategy.
Good luck ... may you get what you are after.