A vendor is a person who is selling a property or a piece of land. However, it does not necessarily mean that person has ownership of the entire property. The best example is a situation whereby someone with a mortgage wishes to sell their home. The property might be worth £500,000 but the vendor still has a mortgage worth £250,000 on that property. As such, the vendor is legally permitted to sell the property, but technically they only own half of it. The lender of the mortgage would own the remaining half.
The vendor can choose to sell their property via an estate agent if they wish. This is still the most popular route to selling property in the UK. There is no legal requirement to go through an estate agent, however. It is possible for a vendor to sell their home on their own by advertising it, managing viewings, and then dealing with the process of passing ownership on to the buyer. However, the process is very involved and requires considerable knowledge to make sure it is done correctly. Most people are happy to pass the work onto the professionals.
Others choose services that allow the vendor to pick and choose which elements they are prepared to do themselves. For example, you could take your own photos, manage viewings, and then get the professionals in when it comes to handling the paperwork, transfer of ownership, and surveys. One advantage of using an estate agent is that your property would likely be seen by more people in the area. Advertising online and in their office would be part of the service they would provide to the vendor.