I'm really glad to see this post & agree with the majority of what has been said.
I was told by a pig breeder with over 30 years experience, that if you don't get a pig 'in-pig' soon after she matures - I'm under the impression that is before she is a year old - that she will start to lay fat down on her ovaries and go infertile. The same can occur if you leave it too long between pregnancies too.
So I was in a rush to get my 2 gilts serviced ( long story), but I currently have 9, 4-5 month old weaners running round! I had plenty of enquiries & my first litter weaned mid October sold without a problem. It is expensive to feed them, but bit by bit I seem to be getting bookings for the meat. Usually, including our own requirements I should be able to grow on for meat about half of the weaners my sows ( now), produce. I was partially prepared for this, but the cost of raising my gilt from weaner to breeding age, meant that in the long term her fertility was more important.
But I am now in the process of adapting my sows cycles to times that are more suitable to selling weaners. So if you are buying weaners to breed from, the time of year they were born is really quite important for any breeding plans you may have.
Personally I have found that time of year for meat sales, makes little difference to my customers. Many buy a half pig - which is a big expense before Christmas, so they may prefer to buy during the year & hold back a leg for a special occasion if they want. I've tried Hilaries Mum idea, but have had no takers yet, though people are interested in the thought they can pay monthly. So it will happen one day!
Good Luck
Time is everything. The longer you are around the more confidence people will have in you and gradually it will fall into place.