A domain that someone else has used is not a blank name with a nicer spelling. It carries whatever its previous owner did with it, and some of that only becomes visible after you have paid.
Find out what it was
An internet archive will show the actual pages the domain served over the years. Look at several dates, not one.
What you are looking for is a previous use that would embarrass you or that search engines have judged: spam, adult content when you are selling to businesses, a scheme of some kind, or a site in a different language and market entirely.
Some of that is reputational and some is a penalty you inherit. Both matter and only the first is obvious.
Check the mail reputation
This is the check people skip, and it produces the worst surprise.
A domain used for spam can be on mail blocklists. You will discover this when your first legitimate message bounces, and delisting is a process with a timescale you do not control. What to do when your IP or domain is blocklisted has the detail.
Check the domain against the main lists before buying. It costs nothing and it is the difference between a name and a name you cannot send email from.
Check whether it is indexed
Search for the domain itself. A previously significant site with no results at all has probably been removed from the index, which is a strong signal.
The difficulty is that a penalised domain and a brand-new one look identical from outside. You find out by trying to rank and failing for months.
Where the domain was bought specifically for existing search value, that value is exactly what may not exist.
Read the registration history
whois example.com
A domain registered for many years by one owner is a different proposition from one that has changed hands repeatedly in short registrations.
The second pattern usually means it has been traded rather than used, and occasionally means each owner discovered a problem and moved on.
Buying safely
The transfer method matters as much as the price.
A marketplace or registrar-managed sale holds the payment until the domain moves, and both parties are protected. Use this where it is available.
Escrow is the equivalent for a private sale and is worth its fee.
A private arrangement with a stranger means paying first and hoping. Domains are transferred by people you cannot compel, and there is no chargeback that gets you a domain.
Expiring domains are not free domains
A domain that has expired does not become available immediately. It goes through a period where the previous owner can still recover it, then a redemption period at a substantial fee, and then it may be auctioned rather than released.
Services that "catch" dropping domains exist because the process is competitive and largely automated. Waiting for a valuable name to simply become available is usually waiting for nothing. What happens when a domain expires sets out the stages.
After you own it
Check the registrant contact is genuinely yours and verified. An unverified contact address suspends the domain fifteen days later, which is a poor start. For that, see unverified contact details.
Enable the transfer lock and auto-renewal immediately. A domain you paid a premium for and then lost to an expiry notice nobody read is a real way to lose money. See domain locking.
And if the previous site had inbound links worth keeping, map its old addresses to yours rather than letting them all fall to a homepage. Planning redirects goes into building that map.
Look at what the domain used to be
The archives hold what the registration record does not, which is what the name was actually used for.
Search an archive service for the domain and read a few captures from different years. You are looking for one of three things: an ordinary business that closed, a site in a category you would rather not inherit, or a page of unrelated links, which suggests it was already being used to sell links.
The last is the one that matters most, because it usually accompanies the link profile problem below. A name with no captures at all is frequently the safest, since it never carried anything.
Read the links pointing at it
An expired domain is usually being bought for the links, and those links are also the risk.
What you want is a modest number of links from sites in a related field. What should stop you is a very large number from unrelated sites, many in other languages, many with the same anchor text repeated, or a profile that looks bought rather than earned.
That pattern does not become yours to fix later. It is attached to the name, it arrives with it, and disavowing at scale is work with an uncertain result. Where the links are the entire reason for the purchase and the links are poor, there is no purchase worth making.
Check the trademark before the price
A name that resembles an existing brand can be taken from you after you have paid for it and built on it.
Search the relevant trademark registers for the term, in the countries you will operate in, and check whether an active company already trades under it. This costs nothing and takes ten minutes.
The dispute process for domains is deliberately cheaper and faster than a court case, and a holder with a registered mark and a name registered after theirs is in a weak position. Discovering this after the site ranks is considerably more expensive than discovering it before the bid. Choosing a good domain name goes into the wider selection.
Where the money is at risk
Aftermarket purchases fail on the transfer rather than on the name, and the failure is expensive because it involves a stranger.
Use an escrow service for anything beyond a trivial amount. The service holds the money, the seller transfers the domain, and the funds release only once the transfer completes. Paying directly and trusting the transfer to follow is how people lose both.
whois example.com | grep -iE 'registrar:|status|created|expir'
Before agreeing, confirm the domain is not locked in a way that prevents transfer, that it is outside any window where a recent transfer blocks another, and that the seller is the actual holder rather than somebody offering to acquire it. Transferring your domain deals with the mechanics once the deal is agreed.