Clarke and Carter would probably be a good broker to approach to ask for a guide price range - apart from the PSSA website, they have handled more Seals/Parkers over the years than any other broker. As Brokers they will also have access to a trade data base that tells them what previous boats of the same make/model have actually sold for.
But by my personal observations, asking prices for Seals/Parkers, which had held up far better than other better know brands for several years after the financial crash [probably due to scarcity of choice in lift keel yachts] have fallen sharply in very recent years [>30%], and the time taken to sell has markedly increased.
The most recent 325 that I noted advertised was asking £44.5k in 2016, and had been progressively reduced from £61.5k since 2011 ....
Geoff Sheddick
Parker 27/146 "Stroller'