Register both the .co.uk and .uk
If you can offer a buyer the .co.uk version of the domain and the .uk version (e.g. widgets.co.uk and widgets.uk) together it is the ideal package for a business or domain buyer. This means they will not have the hassle of negotiating again for the alternate domain from another seller. This will almost certainly mean you can demand a higher price for the domains by selling them together as a package.
It may be the case that the buyer is only interested in the .co.uk version at first, but if you can offer the .uk version for a nominal extra price e.g. £100 – £500 this may score you an extra couple of £100s on top.
Register social media names
Most businesses will need to register their business username on Twitter, Instagram, Facebook etc. If you can pre-register the names and offer these together with the domain names this can really add value. Especially for a new start-up looking to by a brand and get started selling their product as soon as possible.
Create a logo and brand
A simple logo, tagline and colour scheme to go with a domain can really add a sense of purpose to a domain. Instead of seeing just black letters on a screen with the a pricetag. The buyer sees the domain as a business and product with real world potential.
Create interest and traffic
Give yourself the best chance to get the highest price for the domain by offering the domain to the right market. You want potential buyers that will be interested in the domain. This can be done by advertising the domain on community forums, contacting existing businesses or dropping some DMs on social media. E.g. if you want to sell Bookshop.co.uk go and email some book shops and let them know.
Be patient and wait for the right buyer
Don’t rush selling your domain name for a quick profit. If you really believe your domain is worth the money then hold out for the right buyer. It can be very tempting to sell early for a quick £500 or £1000, but hold out and find the true value of your domain in the market.