Any shop that buys your kit will never pay you its full second hand market value. You'll get less because the shop is just buying stock; so they've got to turn around and sell it themselves and for profit. Now what a shop does offer is a fast, simple, easy and safe way to trade.
In contrast if you use forums,
ebay, local camera club sales and craigslist you've got to hunt for the sale yourself. You CAN get more money from the sale because you're doing it direct, but you've got to put in the effort to find it in the first place. This can take time so it can take longer to sell.
There are also some risks, although you you play it safe you can generally avoid problems or at least be protected. Good methods include:
1) Trade with known individuals on the medium you're trading on - forum members who've been around for a while and who are active; people with a good trading history on
ebay.
2) Avoid possible problems; people with little to no site history; people who want to do odd things like ship to a different place than is registered on their account/card; people who want to pay you WAY more than what you're clearly advertising for the item; etc....
3) If on Craigslist or meeting up from a forum to trade always meet during daylight hours in a public place. A cafe or restaurant is a good option; parks are not so good and anywhere private or down backroads or way out in the countryside is risky.
4) When posting use protected postal methods. Sure it will cost more, but if you get recorded postage and delivery you've got a clear papertrail to show that you kept your end of the deal.
5) When paying use protected methods - ensure that you or the other party pays the 3% extra on paypal with a business trade; don't use Western Union or direct bank transfers.
6) Keep up good communication - esp if its taking a little while to get things sorted. Most people are patient, but they grow quickly less so if they are left in the dark.
7) Good clear descriptions and ideally photos - point out any damage, wear and tear or problems. You want the buyer fully informed on what they are purchasing with no surprises. Its easier for you in the long run rather than dealing with hte fallout.
8) Remember you don't have to trade with anyone; if you get bad vibes or feeling during the trade just make your excuses and walk away from the deal.