While you can get a pretty steep discount on a former charter boat, I wouldn't recommend buying a boat that way. Most charter boats are in significantly worse shape and have significantly more abuse and use than a privately owned boat of the same age. Also, many times, the equipment on a charter boat is the minimum that you'd want on a boat that size and generally needs to be upgraded a bit for personal use. Finally, the cabin layout and such is often optimized for the charter business, and not as well suited to personal ownership. For instance, many will have a three cabin layout, with two aft cabins that are relatively small compared to the non-charter version of the boat which has a single, much more spacious aft cabin instead.
Sorry but I disagree.....Many charter boats are better maintained than non charter counterparts. With a decent Marine Survey and sea trials you could very well find a great buy. You will also find some bad ones but that is what a good survey and sea trial is about after you down select based on you preliminary findings.
The season for boat coming out of charter run about the same as the end of charter season. That will be different for varying locations.
The Caribbean's high season is from about November until March then downward until October and September. Most charter company's have owners accepting boats back after the charter term of something like 5 years in a time frame from July thru September. NOTE: Recent released boats from the larger charter fleets have contractual requirements for certain refurbishments and repairs. Almost anything your survey finds as a problem will be fixed with very little discussion... they have to fix it to release the boat.
I've owned one purchase like this for several years and I know many other cruisers both part time and full time that got great deals on boats that fit their cruising needs by purchase from charter fleets. You just have to select and inspect... same as you would for a new boat or one that may have been non fleet owned and probably been setting unused for long terms. Most mechanical systems including the engines, hate not being used more than being frequently used. The biggest problem you may find on a few fleet boats is lesser cosmetic condition. Many are almost like new and well maintained with log books to show exactly what has occurred in the way of replacements and maintenance.