Ahh the common aerial problem, and thankfully an easy fix (usually!) If you look at the base, you'll see the top half has 2 small holes on top either side of the mast, get a pair or long nosed pliers or similar, locate in holes, and unscrew the collar, may be worth covering the boot and wing with a thick blanket etc to save the paintwork just in case you slip! Once that's unscrewed and removed, have an assistant switch the radio on, so aerial starts to go up, but keep pulling , the whole mast should come out, and spew out a length of toothed plastic too. Once it's all out clean thoroughly and liberally spray with WD40 or similar until spotless, make sure there's no dents or kinks in the mast when fully pulled out, if there is, replacement masts are available for the £17 mark from Rover (or were) and the odd decent car audio or car spares place should do them too.
Once happy with state of the mast, feed the plastic back down the hole, and have someone switch the radio off, as the motor starts push plastic bit down, it should then catch on the cog inside, as it does, feed the mast back in, and finally make sure it's fully down, replace the chromed nut, and switch radio back on, aerial should then go all the way up, and *SHOULD* fully retract again once switched off
The teeth on the plastic aren't brilliant, and over time as the grease dries out, and dirt gets in, the cog slips on the teeth as soon as any resistance on the mast is felt, be it a build up of dirt, or a slight kink in the mast, so next thing you know, it's not fully extending or retracting.
HTH and good luck with the sale