Sticking a sign up at the gate isn't a promising way to go. Before we kept our own sheep and saw that kind of sign, the questions we asked ourselves were: How much is half a lamb? What cuts will we get? Will there be burgers, mince and sausages? How much space will it take in the freezer? How long would it take us to eat it? How does the price compare with lamb from the shop? This must be much cheaper surely? How do we go about getting the lamb? Just walk up to the door or will we have to go round the farm and find someone? Is it frozen in joints or do we have to chop it up ourselves? (well, people really don't know) Is this supplier any good? And so on.
Have you thought about advertising online? You can answer those sorts of questions and more, and explain to people why your lamb is special. Did you research the breed for meat sales before you bought them? We used to sell whole lambs to Mr F's work colleagues and regularly sold half a dozen, but no more. The rest we ate ourselves or gave to family members, joint by joint, or as burgers and sausages (very popular). Our breed is Hebridean, so a Primitive which is something of a niche market, and sometimes mentioned by celebrity chefs, so that is a good selling point.
The whole trick about selling anything is salesmanship (I don't have any!). I see people selling things for outrageous prices, but because they have the patter people are queuing up to buy. Do some research online looking at what others do to advertise their product - sometimes it's just a load of Blarney, but people buy - I've seen Hebridean meat sold at twice the price for half a lamb that we charged for a whole one, so four times the overall cost (and Heb carcases are quite small) So charge more and let people know you have a specialist, superior product. Get good packaging and sell through the post/delivery service.
You have been doing this for only 18 months - in this malarkey you don't just walk in and become an instant success. You will learn so much from experience and from others. In a few years' time you will be laughing that you thought your progress wasn't quick enough so early on. So relax, take the time to enjoy what you're doing, and keep your expectations low - then you won't be disappointed