The first thing to check is their business on Companies House, if they are not on here or they have recently registered. Be cautious.
So if their website says they have been trading for 10 years and the business was only registered last month on Companies House, that's a red flag.
Be honest with yourself, do you really think you can get a £15,000 garden office for £3,000? We have come across some great shipping container companies that are cheap, but not that cheap!
Check WHO you are paying, if it is a genuine company, you will be paying into their bank account, not a personal one.