I would think about now, being the beginning of summer and good riding weather, but what are my chances of selling a 14.2 Palomino mare in September time?
It always used to be spring/early summer - you can tell by the thickness of the H&H horse for sale section!
Having said that I would consider buying something later in the year if it meant I could get to know it during the winter months and be ready to compete in the spring.
Now is the best time to sell, because depending on which disciplines you are in you have the whole competing season coming up. Plus you don't need to put as much energy/expense into horse care as you would over the winter.
Late Spring is probably the best time because it is coming into the summer - less work and expense, nicer weather, competition season and summer holidays. I'm sure you would sell her in September though as a lot of people like to buy at that time of year to get used to the horse over the winter. People buy horses all year round, I just think you would sell her more easily and possibly get more money for her now.
this is as good a time as any i find sept wont be as good i have had one advertised for ages then this last two weeks had loads of calls and sold to 1st to see her