JMHO : regarding the two items mentioned in your listing, without a detailed description of each accompanied by photographs, any valuations offered via the BBS are speculative (and in any case more credible if from potential buyers rather than someone who's got a similar item already where their ownership adds provenance and additional value).
Basically cased is good and if both are in fwo and pristine condition I wouldn't be surprised to see an asking price of around the £300+ mark for the FWB and maybe £225 for the Diana being reasonable.
In lesser condition with lots of signs of useage and problems with function, less than half of those prices. In my experience both of those items suffer with Seal degradation and high servicing costs.
In addition, whilst they are quality manufactured items, there's little by way of scarcity value in either unless in vgc and at a guess if you visited any of the established Match Air Pistol facilities in your area you'd find quite a few members in each club that you visited with examples of both.
A starting point for a "lowest price" on both might be provided by contacting a few local Gunshops and asking for both a buying in and a commission sale price. Auction prices are a help but speculative as it all depends on the day.
Regarding where to sell : you could try an On Line advertising site, or try listing them on here in the For Sale section, or maybe even an On Line auction site.
HTH, Vic Thompson.