Unless you're dealing with someone who wants the gun and mags for themselves and will trade you the QA, the best way to do it is sell the stuff privately first. A store will offer a whole sale price on the gun and mags. The offer will vary depending on condition of the gun and mags, area, and what they think they can get for the mags.
Having literally been all over the country, you should expect an offer of $300 for the pistol (& 2 10 rounder mags), assuming good condition & coming complete with case and accessories. It's low and you will do much better in a private transaction where you can expect to get around $400 for the pistol (& 2 10 rounder mags). Night sights and High-cap mags boost the price. -With two High-caps you could probably get near $525 in a private transaction. If the person is not getting 2 10 rounder mags, you have to discount the high-caps a little, since they normally would get the gun with them and are included in the price. -This is all assuming good condition and the most current frame model for the Glock. Some people won't pay as much for a Glock with the older frame styles, the ones without the finger groves or accessory rail. That said, some actually will seekthem out since they feel they get a better grip without the molded finger grooves. -It's a highly personal choice. You may want to sell the mags separately and replace them with 10 rounders if selling to a dealer. -Basically, you won't get a straight trade from a guy with an FFL unless he's getting the gun for himself.
As for prices with the mags, I've seen Factory G21 High-caps, drop-free, go for $125 in Maryland and as low as $75 in Washington (state). -Unless you're selling to a store that sells them at around $90, I wouldn't think that they'll give you more than $70 per mag. -A private person is another matter. -Also the prices quoted are for new mags or like new mags. A mag that is used and really looks used will be less. Similarly, non-drop-free mags will be less.
The QA's when you can find them new, will run roughly around $530. I've seen some go for less (sub $500- rare) and others go for more ($550-$600). Others may disagree, but I'd say anything between $525 and $560 is an acceptable price. -If you're paying more, it is generally a premium so you can have the pistol now as opposed to spending your time looking online for the next good deal. However, at a certain point, your time is worth more than the $25 - $50 you may save.