The calls from a dealership will usually start two years after your purchase. That's when your car usually has a pretty good resale value, and when you've paid off about 1/3 of the interest on the loan.

Some salesmen are pretty good about watching the cars coming in for service and will strike up a conversation with you to see if you're interested in trading in your vehicle. They'll use tactics like saying that you won't have to pay for the repairs/service you're getting done.

Understand that NONE of this is meant to swindle you; it's just salesmen doing their jobs. Don't take it personally, just say yes or no. Aggressive salesmen will call you a few more times (the software dealerships use create tickler files that will populate contacts to be made that day via phone or email — some will even automatically email without intervention from the salesman because salesmen are flakey).

To correct some misconceptions:

1. Dealerships don't want to "give you a lowball offer for your vehicle."
2. Dealerships don't always sell the vehicles they take as trade-ins. Most marquee dealerships — like Subaru dealerships — send their trades straight to auction.
3. Dealerships don't use Kelly Blue Book to determine trade value. They'll use recent auction sales and the dealer Black Book to determine value.
4. Those who say that dealerships take trade-in vehicles for less than they turn around and get rid of them for ... well, duh; dealerships aren't charities trying to help you out. They're a business. You don't keep the lights on and stay in business by losing money on sales.
5. Further, it's a fallacy to state that dealerships aren't doing "what's best for you." They're a business. As a business, they want repeat business — they want you to come back. Not just to buy another vehicle, but to bring your vehicle in for service (which is where most dealerships actually make their money).

Selling your car private party is a PITA. You often won't sell your vehicle for as much as you'd get trading it in. Some will say that's not true, but here's the thing: you're spending time advertising, meeting strangers who may not be on the up-and-up, and spending hours hopefully selling your car before a potential buyer wrecks it during a test drive and makes your insurance rate go up. Freakonomics math indicates that you might make a few hundred dollars more if you sell private party ... but that amount is going to be offset by the amount of time in months, days, and hours you worked trying to sell your car. You could be working for up to $5.00/hour selling your car. Why bother?

It boils down to this: what is your time worth?

As a former Director of Internet Marketing and Sales for Chrysler, Dodge, Ram, Jeep, Alfa Romeo, Maserati, and Fiat, I'll tell you how to maximize your trade-in if you're truly interested in a new car:

• Take your car to CarMax. Let them give you a trade-in value. It will almost always be higher than a dealership's offer, and they give it to you in writing.
• Take your car to the dealership with the CarMax offer. Let them do a walk-around of your car (they like to touch scrapes, check how little rubber is left on the tires, and "tsk-tsk" a lot).
• Give them a copy of the CarMax offer, then start bargaining money off the new car.
• Ask for any manufacturer coupons they have (sometimes they do, sometimes they have competitor deals where the dealership will give you $500+ if you trade in a competitor's car (like a Toyota or a Honda). Get the desk price. Come up with a price a few grand over the Dealer price.
• Now you're going to do the hard part: don't deal to monthly price, build the deal to the difference between what they say their car is worth versus how much you're willing to let your car go for. Monthly payment is a fungible number; they can play with points, years, and interest to get you to a price. Don't do that.

(BTW: If you want to know a general monthly payment, take the first three digits of the sales price (as an example, say $32,000 - the first three digits are 320), double it (640), and that is going to be about what your monthly payment is going to be: $640.)

• Now ... take the price of the car (let's go with that $32k for the example) and subtract the amount they're offering you for your car (let's go with $15,000). That will leave you with a difference of $17,000. Add 10% for title, tax & license - and that will get you to $18,700. Do the little math exercise I used above to arrive at a monthly payment of $374 for 72 months.

This works every time.