Looking at the sites of where I bought my stuff from.
I'll just lower the price a bit for the items that are in good condition and see if they sell, I'll just keep reducing the price until someone buys them I guess (:
You are on the right track to start with a price near your purchase cost if they are in good condition. Other things to keep in mind when selling used items:
Who is your target buyer? Anyone interested in guitars for example, or are there people passionate about your item's manufacturer, model, colouring, or features. Are they new to guitars and getting their first one, experienced and upgrading to a better guitar or getting a second guitar, or are they a collector?
If your items have a passionate fan base you may be able to ask more for it then a new item or an equal quality item of lesser popularity. For example people pay more for old items (i.e. toys like barbie, pokémon, miniature race cars....) due to passion and scarcity then a new one, or because an old model is compatible with their other hardware they are hanging on to and new items are not.
Where does your target buyer shop for what you are selling? If you were in the market for your guitar in its condition, where would you look? Do these places have buy/sell forums or ad services you can use?
If they are someone who would be taking lessons, then check if the places where classes are held in your area have bulletin boards for selling used equipment. You can post ads there with pull of tabs with your number and a URL for someplace you've advertised the items online where they can see more information.
Check your main public transit stops (particularly hubs where busses meet trains or subways) for newspaper boxes that have the local buy/sell ad booklets and free commuter papers (many larger cities have these, but you'll only see them if you get to a transit hub early in the day before they are all gone). Small ads in these print media may help you sell your items.
Does your guitar have a quality or value that other similar guitars wouldn't have? A custom feature, rarity because it was a limited production, or has something been done with it since manufacture that improved it or made it special? For example a custom paint job, was made by a well-respected craftsperson, was of a limited run of replica guitars of some famous rocker, or was at some point owned or played by someone famous?
Keep in mind that the special quality of your item can help or harm you with your market and needs to be considered when deciding where to advertise. If you repainted the guitar a bright neon green with black lightning bolts it may look really cool to you, but anyone who wanted the original finish won't be interested. In that situation you would advertise to people who appreciate that special quality, i.e. to the modifiers and tweakers, not the purists.
Do make sure you disclose any modifications or changes since manufacturer that might affect the use or performance of the items.
What state is it in? Are, or had there been, any scuffs, stains, or previous repairs? How old is it? Someone would be more likely to pay closer to retail for an item that is "like new" or "gently used" then one that has been "well loved" (unless age or use helps improve the item, like "seasoned" cast iron pans or aged whisky).
Make sure to take good, clear pictures. Tiny ones taken in a dimly lit room may hurt you because people would wonder what you are hiding. Take shots of several angles. Showcase the special features and if possible that working parts are in good condition.
Try placing a solid colour (ideally white) sheet on the floor of a room that gets the sun during the day, lay the item on the sheet, and then snap a picture from above. Or, drape the sheet off of the back of a chair and onto the floor like a photography studio back drop, stand the item up, and take the picture from a bit of a distance.
Try to keep out as much of the surrounding room from the pictures as you can unless your home is super tidy and/or you are rich. Seeing the room would result in assumptions and judgements on the part of the customer, as well as unconscious association of the item with a lifestyle. I.e. if you are rich and tidy and the room shows this, then seeing the room would make the customer feel that it was well taken care of, picture that lifestyle, and feel that buying the item would be a step towards that lifestyle. If you are not well off or if the room is a mess they'll think this is an item that only you buy when you don't want to spend a lot of money, or it's been in an environment where it may have been abused and you aren't disclosing a flaw. For example think of the difference in the advertising of a new sleek commuter cars or sporty off-roaders, and the advertising of your local used car dealer selling previous models of those same cars. In the former you see the car breeze along the highway, passing flashy business buildings, or easily maneuvering through forests. In the latter the car is sitting on the lot with a balloon tied to the antenna. The used car ad doesn't feel as cool or adventurous.
If you can take a video of it, this is a good idea. Especially if you can demonstrate that it is in working condition in a video. Upload the video to youtube and embed it in your online advertising.