I think its not easy to find second hand quads in Cape Verde.
If you find one have in mind that the price of a new one in Cape Verde should be always about 30% – 40% above the price in Europe because of transport costs and import taxes. So the used price will also be about the same percentage about the European market, plus the fact that there is not much offer, so price is relatively high.
I know from somebody who bought one for about 5.000 EUR, it was a Yamaha one with 4×4 and about 4 years old and not in a very good state…
Hope this helped !