The answer really is "0". Buy the bike new, don't ride it at all, and then after 5-6 years you'll be able to sell it for top dollar. Not what you paid for it, but lots of people want new bikes for cheap so it should sell real quick!
Really, there is no straightforward answer. Stokedcrf touched on it perfectly. It depends on the bike and the market. If the bike isn't desirable, it won't sell. If the bike is a market where not a lot of people ride, then it won't sell. There's too many factors that go into the pricing and something is only worth what someone else will pay for it.
If you're considering selling a new bike after just 5,000miles just to get another one, than trade it in. Trade-in makes more sense in that case. You can make more selling it outright yourself in terms of "cash money". But so many people forget that they aren't paying interest on a loan if they finance it on the difference and you don't have to worry about doing the work on it. Time is money! And the difference in price typically works out to a couple hundred bucks difference between trade-in and private party when you factor in everything. Not worth it IMO if you're just going to buy new and get rid of it so quick.
Just like cars, the cheapest one you'll ever own is the one you have right now!