A domain can be perfectly available and still be a legal problem. If your chosen name is the same or confusingly close to a registered trademark in your field, the owner can ask for it, and in a dispute you may lose both the name and the money spent building on it.
The registrar will happily sell you the name. Registration is a technical act with no legal check behind it, so the check is yours to do, and it is much cheaper to do it first.
Availability is not permission
A domain registry works on first come, first served. It asks whether the string is taken, not whether it infringes anything. Trademark law works differently: it protects a mark for particular goods or services, in particular places, against names that could confuse customers. The two systems overlap only when a dispute begins.
That is why the same word can legitimately belong to different businesses in different fields, and why a domain you register can still be challenged by one of them. What matters is likelihood of confusion: similar sound, spelling or meaning, in a related market.
Search before buying
Before buying, search the trademark registers for your country and for the places you sell. Most countries publish a free searchable database. In the UK it is the Intellectual Property Office, in the EU the EUIPO, in the US the USPTO. The World Intellectual Property Organization also runs a global brand database that covers many countries at once.
Look for the name itself, plausible misspellings and the same word in related industries. Search the sounds as well as the spelling: a name that is spelled differently but pronounced the same can still be confusingly close. Then look beyond the registers. An unregistered business may still have rights in a name it has used for years, and a quick web and app-store search will find most of them.
A worked example
Suppose a village bakery wants orbitbakes.com. The domain is free. A register search shows a registered mark for "Orbit" covering software, which is a different field, and a second for "Orbit Bakehouse" held by a bakery chain two counties away. The first is probably harmless. The second is the problem: same trade, similar name, and a customer base that could easily overlap once the bakery starts delivering. The reasonable move is a different name, or a short conversation with the chain before the oven is bought. Nothing in the domain registration would have warned anyone.
Which names are safer
A name that is a common dictionary word is usually safer than an invented one that resembles a famous brand. A shop selling bread can usually use an ordinary word for itself. A made-up name that borrows half of a household name is asking for a letter from a lawyer, particularly if it is in a related field. Descriptive names are easy to defend against, but also hard to protect for yourself; choose with that in mind.
When to pay for advice
If the stakes are high, a short consultation with a trademark lawyer is cheap next to a rebrand. High stakes means that you will spend real money on signs, packaging or software built around the name, that you will sell internationally, or that the search turned up anything close. Take your search notes with you. Many lawyers will give an opinion on a short list for a fixed fee, and a fixed fee is a small price against a forced change after launch.
Look at your own configuration
Make a table with your name, three misspellings and two synonyms. Search each in the register of your home country and each place you sell, and note any hit in a similar field. Save screenshots with dates. Searching again before launch, and again a year later, takes a few minutes.
Common questions
If I own the domain, do I own the name?
No. A domain gives you the address, not the right to trade under a name that someone else holds rights in.
Should I register my own mark?
If the name is central to the business, usually yes. It costs money and time, but it also gives you something to enforce.