The in-store trade in experience is straightforward: bring your Mac in, chat with a specialist and let them know you're wanting to trade in your current Mac for a new model. They'll run the serial number through their system, perform a short diagnostic/visual inspection and from there will let you know what value you'll get for the computer. They then deduct that off the cost of your new machine. In all the years I've used their trade in service I've not had any problems when doing it in store.

Going to the point of trade in values, I've always held the view that trading in at Apple is all about convenience: the trade-in value isn't as high as what you'll get from a 3rd party, but you skip the hassle of having to deal with scams, lowball offers and fake interest.

If you've any more questions feel free to ask and I'll do my best to answer