Well firstly, Welcome to Acorn Domains.
I'm not sure how much you know about domains, but basically.........
Getting an idea of prices is a bit tricky. A good place to start is to think of some short meaningful names that you might want to call your site and see if they are registered yet. I'd recommend using the search boxes on the home page of registrars 123-Reg.co.uk or 1and1.co.uk.
However, all the best names are taken, but if you are happy with 'brandable domain names' like DavesAds.co.uk or OurRutlandClassifieds.co.uk (two names I chose at random) you might have a better chance. Just be prepared to spend money marketing sites with this sort of domain, because they won't get much natural traffic, at least to begin with.
The next stage up, and one that I'd recommend if you want to have a popular, and possibly profitable site, is to buy a generic domain that is more of an investment. This would be a shorter, keyword rich descriptive domain that will also help your site rank well in the search engines - "free ads", "Classified ads" etc.
If you are looking for short, descriptive 'generic' names you are going to have to pay. The better classified names don't come up for sale that often. DomainPrices.co.uk, part of the AcornDomains family of websites, has a good searchable database of UK domain sales over many years. You can search this to get an idea of .co.uk sale prices. NameBio.com (not connected with Acorn) is a similar one for .com domains.
After you have had a good think why not start a new thread here stating more precisely what you are after (i.e. .co.UK only or whole world site, free and paid ads or free only, the theme or angle that makes your site better than the others - that type of stuff) and how much you want to invest.
As a rough guess you will get plenty of offers with a budget of up to £1,000 but expect to pay much, much more for top quality domains. A third option is to trawl somewhere like the domains for sale site Sedo.co.uk.
I suppose the only tip I can give is do your homework on domain prices before spending money you cant afford to lose.