Firstly, here's the full documented law on art copyright, I'll be going over what much of this says in the context of this guide, but it's worth reading in its entirety if you don't understand anything.
-> https://www.writersandartists.co.uk/artists/advice/169/ an-artists-toolkit/essential-information/ copyright-law-for-artists
In the terms of what is lawful, there is only one thing you need to do to protect yourself from fraud, or theft, and that is make sure you are explicate! A chat over Skype counts as a written contract, and it's worth recording the logs of such discussions. But what should you be explicate about?
Well there are three main terms behind the sale of pixel art, they are, from the article above -
There are three common categories of sale for copyright and reproduction rights:
Artists sell copyright outright. They have no control over the ways in which images are subsequently used, and the new owner of the copyright is free to sell licences as they see fit and to retain all the profits. (Sometimes, however, agreement is reached that copyright will be sold but royalties will still be paid to the artist. See below.)
Artists sell copyright for a specific limited purpose; neither artist nor publisher can use the image for any other purpose. For example, if the image is published as a limited-edition print it can never be used for anything else, either by the artist or the print publisher.
Artists sell reproduction rights, or a licence, for a specific limited purpose; the artist retains copyright and can continue to profit from it. For example, the artist sells the image for use on a set of table mats, but can then go on to sell it for other uses.
These are the things you should be explicate about, to summarize them in a less lawyer gibber jabber way, they equate to this:
Artists sell copyright outright. This is most common term agreed upon via "sale". I sell you my art, I have no longer any right to it, I can't sell it again, and I cannot use it again.
Artists sell copyright for a specific limited purpose. This one is less common, it's where you lose the rights to your art, but the person you sold them to can only use them in a specific way. For example, I sell you an art pack of tiles, but you can only use them within a single project we pre-agreed on. Just like above, I lose all access to the art and cannot use it for myself.
Please read this all very carefully, because wording is very important! As I previously mentioned, the key to being secure is to BE EXPLICATE!. For the sake of argument, it is generally agreed upon that a conversation that contains simply the details "I'm selling you this bit of art for x" equates to the same as the first agreement option. Where you lose all rights, however this COULD arguably be disputed in court. It's always worth it to be vigilant, record chat logs, and one more time, BE EXPLICATE!