It is Summer( in Northern hemisphere). You will see many riders on the road. It is natural that used bike price goes up when the weather is nice and sunny. Those riders on the roads are free advertisement for motorcycle industries. In other words, you get better deal buying bikes when it is cold, wet and miserable. It is true. Old bikes are hard to start up when it is cold and less likely to be able to provide test rides which makes it hard to sell.
Looking through ebay and Craig's List, chance of encountering a good or even a decent deal gets slim during the Summer time, but some people list those bikes extremely high price like buyers are supposed to be dumb and bound to get ripped off. I am glad that those guys are proud of what they got but some listing is often down right ignorant or even insulting. Custom bikes that hastily put together with half-ass finishing job is not considered chopper or bobber.
Yeah, I understand that you spend so much time and money on the bike, adding often completely unnecessary after market parts and flaming paint or get ripped off by local motorcycle repair shops, wasting more money on the bike but that doesn't mean you are increasing the value of it. I see some pretty embarrassing, esthetically sad looking "AWESOME CUSTOM" bike asking for too much money.
Most of the time, motorcycle is not considered as a good investment. You are likely to lose money on that. Unless you are ethically challenged fuck head who buys crap bikes for dirt cheap and list the bike with at least 20% mark up while you don't do anything to it. You might also go back to the seller's garage where you have been once before when you picked up the bike and steal what the seller's got.
There are a few sites that tell you what is( a guide line) value of your bike such as Kelley Blue Book, which lists both trade-in and suggested retail value. I also find NADA guides very useful since it gives different pricing depending on the shape of the bike. It also lists detailed information on what shapes of bikes are considered as in "Excellent" , "Very Good", "Good", and "Fair". I see a lot of sellers putting their bike's price more than "Very Good"price, but should be in somewhere between "Good" and "Fair". No one likes to be ripped off. You should not rip off others. It is not a good experience to deal with low balling baboons or greed.
I would like to be always fair and even.