I don't think they are so popular with us guys, most are Powell's conversions. I've owned two good ones, a Riviera OXO and an Aristocrat Esprit OXO, both Powell conversions which were traded to me from a working amusement arcade in Pagham in exchange for two Ballys. The third, another Riviera, was quite tatty with blemished chrome work.
I would still buy one at the right price because they are quite good fun in the home as a money box but the factors I would look for are the quality of the chrome work that is whether they still have chrome. Many have been stove enamelled with a hammered grey/black colour. They do sell well to those who aren't concerned about authenticity of look/coinage etc and I have shifted them very quickly.
I sold my Riviera on 10p play for £350 around five years ago. It was exceptional condition and could be set to 1 or 2 play per 10p, it had a £4.00 jackpot.
The Esprit. also on 10p play, sold for £325.00 also in great condition, gleaming chrome. Even the tatty one fetched 275. Both, either on 2p or 10p play can be temperamental on payout due to the later coin thickness changes. You will get short payouts and with my machines' constant jams on coin slides on the higher wins due to the thickness of coins. More likely fixable by some filing.