If you really want to do this, nominet is the best bet. Even though you'd have to sell at least 100 domains each year. You don't have to compete with 123-reg, they sell thousands upon thousands of domains each year.
As long as your priced well, i.e. not seeling a domain for 30 quid +, then you'd be in business. Only thing is getting a large user base, unless you already have one interested in buying domains.
If it works, it's valid.