If you target markets where the native language is not English, you are likely to encounter letters which are not part of the standard Latin character set. It is immediately obvious for languages such as Japanese or Arabic but common as well in French, German, Swedish, etc...
You may be tempted to consider buying IDN (Internationalised Domain Names), which allow characters not part of the ASCII set.
There are several
ways to make money from domain names. The most well know methods are domain
parking, and obviously selling domain names. But if you have a portfolio
containing generic domain names, you may have a third option, known as 'domain
leasing' or 'domain renting'.
Domain leasing allows you to generate
revenue from your domain, while still holding on to the rights. This can be a
great option if you suspect that your domain name is going to appreciate in
the next few years and you are not yet ready to sell.
When registering a domain, try to go for the spelling your market is going to use. This is especially true of domains where the spelling is different depending on the country, for instance behavior (American) and behaviour (British).
Regarding the use of plural, the same applies. What are people more likely to look for: travel or travels ?
When in doubt, simply register all the variations you can think of and track the results!