I'm building a smart plug and I want a circuit to convert from 220vac to 5vdc so I'm using a charger circuit (image source) to deliver the 5vdc I need for the wifi module as it is with no changes made. Can I sell this product under the right terms?
Given that that specific charger was designed by Apple, no you may not. Also bad karma to not credit picture to original source here.
You bought the hardware. You can modify and sell it. I don't think there's any IP load on this...if it's patented, you bought the rights along with the hardware (not to duplicate it, but to treat that piece of hardware as your property).
If you want to resell it, that's your choice. If you modify it, it's best to try to eliminate the original logos and trademarks from your product, since you don't want to misrepresent your modified product as someone else's (they probably won't want to assume responsibility for any negative publicity or liability you might encounter).
FCC/CE/UL approvals may be necessary for you to sell it into certain markets. You will need to requalify the modified components.
If you're only making a dozen for your buddies, likely nobody's going to worry about it unless something goes horribly wrong. If you're advertising it for sale, you can expect somewhat more attention from authorities.
It depends on how many you're making.
If you're just making a few of these units for yourself to use or some of your friends in a non-commercial purpose. No one cares what you use, as long as you're not commercially selling this product or making money off of it you can use an Apple charger or whatever else you want to put in it.
If you are making thousands of these units and selling them for money in a commercial setting, then you must not use someone else's design without permission. Making a 5 volt power source such as a USB charger is trivial today. having someone integrate a circuit similar to that USB charger on to your custom PCB for your Wi-Fi module would be cheaper in the long run then packaging a premade USB power source.
As far as taxes etc goes, if you're manufacturing thousands of these it will probably be done in China and you will never pay an import tax on individual components because they won't come to you. a Chinese manufacturer would purchase all of the components needed and either ship you or a distributor your final product. I don't know what country you live in but if you are receiving a product that is to be sold again, you may not even need to pay import taxes on it. If you are the one designing and selling it.
If you mean you want to embed the Apple charger electronics inside your product, You can do what you want with things that you own.
They probably don't want their name associted with your product. it's not like they are going to provide warranty support for such modified products, nor do they want to be held responsible for any problems your modifications may cause.