Have a look at exdem hifi - this is a website which captures all, or most, of the current deals, both used and ex-dem in most UK dealers. Haven't a clue about the Pembrokes (what price were they? Were they above or below the highly regarded Severn floostanders?).
Once you've established a guideline value and want to sell privately, Ebay is always an option, but fees are punitive IMO, plus they double dip with both Ebay and PayPal fees knocked off your auction. If you go down the Buy It Now route, you might as well opt for a classified ad anyway and see what happens. At least you know if it sells, you'll not be hit for a fee and if it doesn't you always have Ebay or Amazon as a fallback option.
Part-ex is also an option, but you'll get a better rate selling privately. Get chummy with a dealer though and you might get a generous deal offered (I know that Retro Reproductions in Edinburgh pretty good with their trade-in value for instance).
Horses for courses; Ebay is popular true, but does as much to drive a value down, so if you DO go down this route ultimately, ensure you have plenty of photos that highlight the condition of the speakers, talk them up, include any links to positive reviews you find on the web . Sign up with Auctiva, they offer Ebay sellers some benefits for free you need to pay Ebay for (I use them for mine) and you get three pictures for free whereas Ebay charge you after the first one.
Hope that helps. All the best.
Post edited for house rules - Mods