Well I can't say the best season is summmer :p I will say though, there isn't really any best time as far as I can tell. It's just about finding the right buyer, who is REALLY in need of clients. Depending on what amount of resources they use up, how many clients and the system used (cpanl, plesk etc..) That all will determine the price for it. There is no harm in saying you want to sell and just not selling, if you don't find a good offer for it. You can try again later on always