Well this is a trick question, because you definitely should not limit yourself to one.
As a buyer
As a seller on all these marketplaces I have seen price discrepancies on thousands of items. It makes sense that the price would not be exactly the same, but sometimes it can be more than a Grant! As a seller I have taken advantage of this by selling on one site while purchasing on another. I have purchased from Amazon and sold on Ebay and vice versa while still making a profit after paying fees as high as 15%. When you have more information you are generally better off, so do your research.
If you are afraid of buying things on the internet, Amazon would probably be the safest, easiest place to start. It is pretty easy and there are many benefits. If the item is eligible for Prime shipping you will get it fast, though I recommend getting the $1 MP3/video/kindle credit for No Rush shipping. Also, returns are fairly easily and customer service is very high. One downside is that items must be in their catalog, so you won’t find handmade mittens for your guinea pig.
Ebay buying is fairly straightforward, but there are options for buy it now and auctions. Auctions offer ways to get a good deal through bidding, but you have to wait until it ends which could be several days. There are thousands if not millions of sellers on Ebay so your experience for every purchase will be different. One time I had an overly zealous seller call, email, and message me when the item was available to pay for, shipped, was marked as delivered. Ebay is a bit more difficult because you will need to make a PayPal account as well, but I would recommend setting everything up so you can get better prices.
This site is pretty good but it is definitely ripe with scammers. You must do your own research and really inspect the items you purchase before agreeing to finish the transaction.
Amazon and Ebay offer a good deal of protection against receiving a damaged or incorrect item and not receiving your item. On Craigslist you are on your own after the money has left your hands.
As a seller
Generally I know how much I want for an item, but if not I browse different sites to get an idea of what comparable items are selling for. You are free to list at whatever price you desire, but if it is the same as other’s stuff but priced higher it will likely take much longer to sell or never sell. I tend to list things on the lower end so I can get rid of them more quickly and move on. In addition, I list my items on several marketplaces.
Amazon’s fees will often run high. However, you don’t have to take pictures or write descriptions as you can only list things in their catalog. When you list something they will tell you how much you will receive on the final page before you confirm, which is easy for sellers. Sellers are often more experienced and Amazon holds them to a high standard.
Being an Ebay seller is not as easy as Amazon since you have to fill out all the descriptions and take photos yourself. The format is pretty rigid so you are required to fill out all the information before you can list. There are many scammers for higher priced items, so always make sure to follow the guidelines for Paypal seller protection as well as anything Ebay requires.
Until last week I had been paying 10% to Ebay plus 2.9% + 30 cents to PayPal. I sold a MacBook for $1200 and received $1045 before even paying for shipping. However I just opened an Ebay store which offers a 4% fee on computers which means I would have received $1117 – $72 more! Setting up a basic Ebay store costs $19.95 a month, which would be worth it if you sell more than $333 in electronics a month, or calculate your own breakeven with their chart.
Free to sell, but you’ll likely have to spend more time fielding questions and waiting for flaky people. For this always deal in cash and in person, never ship things or use PayPal.
Wallapop and other apps
Now there are apps which function like an easy to use Craigslist. There is not the same number of people on these apps, so it has been just OK trying to sell things. I find people to be even flakier than those on Craigslist.
Amazon protects their buyers more than their sellers, but fortunately I haven’t run into an issue yet, but I would recommend looking up their terms. PayPal offers seller protection against chargebacks which has happened to me when the buyer was using a stolen card and PayPal covered me against the loss since I had signature confirmation. Anytime I am shipping anything over $250 I get signature confirmation, so I have proof in case a problem arises. Craigslist should be done as soon as you get the cash, but I have had someone call me after asking a question, but no huge issue as I am not trying to scam people.
Buy and sell at your own risk. If you are uncomfortable connecting your bank account or credit card to your accounts, then you should use Craigslist, an app, or just stick to shopping in stores. If you are savvier, you can list where you think you can get the highest price. Always make sure to consider your fees and shipping costs before listing so you aren’t surprised when you get much less than expected. Also follow the rules to best protect yourself against fraud.