Well I sell only on eBay and in tiny quantities compared to LCE and SRS above (evening gentlemen ) so my experience is limited. For what it's worth though I've sold Samsung versions of both the DA12-24 and the D-FA100 Macro on there, and both have fetched prices pretty much exactly in line with what others have sold the Pentax versions for.

I can't speak for the bodies but it doesn't seem there's any particular bias against Samsung glass out there...

The Pentacon is a nice enough lens with a similar feel (in terms of image) to the various Pentax 50/1.7's. If you already have one of the Pentax lenses then I wouldn't bother keeping the Pentacon, if you don't already have one of the Pentax lenses then yes; get an M42 adapter and use it.

I'm keeping my BX20 - it's a nice solid camera but worth so little I'd rather keep hold of it for sentimental reasons if nothing else. Shame I can't use the bayonet lenses on my Pentax in the same way I can M42s...

If it's an MTL (3 or 5) then yes it's M42. The bayonet bodies all had model numbers starting with a "B" (B100, B200, BCA, BX20, etc).

Shame it's so far away - almost worth the drive up there to get a B+W 77mm CPL for 20 quid!

As John says the MTL5 body (even a good working one) is probably worth no more than £10-20. The 135mm lens is probably in the same sort of area, maybe even slightly less. M42 extension tubes are virtually a giveaway item so no real value there either.

The interesting bit is the lens. Prakticas of that era could be had with a choice of four different 50mm lenses as "standard" and the second hand values vary wildly. Cheapest was the Meyer Domiplan 50/2.8 (probably worth under £10 now), then came the Zeiss Tessar 50/2.8 (worth around £25-30 now), then the Pentacon 50/1.8 (I sold one recently for just shy of £50) and finally - for well heeled Praktica buyers - came the Zeiss Pancolar 50/1.8 (Chinese bidders on eBay go mad for these and on a good day you can see them fetch as much as £80-100).

So the value of the kit as a whole rests heavily on which 50mm lens is attached to the front of the camera, and that lens will fetch a higher price on its own than it will with the camera attached to it (buyers want the lens, they don't want to pay postage for a body that will in all likeliness be consigned immediately to the nearest bin or charity shop).

Your shot of the tulip shows the other thing I like about this lens - the bokeh is very "painterly", like someone has filled in the background with a very broad brush giving an impressionistic feel to photos. Lovely stuff

I picked up a copy of this lens a couple of weeks ago as part of a larger kit and expected to be shifting it onto eBay shortly thereafter - after a few days with it though it's rapidly becoming one of my favourite lenses and my FA77 is looking worried...

I find it's particularly good at reproducing reds. I've been shooting the tulips in the garden and I'm getting a vibrancy that, while not quite true to life, is simply sumptuous! The contrast seems a little lower than my DA lenses so maybe that's playing a part? I've not taken any people-portraits with it yet but I've got a feeling it'll be a cracker.

You can get ready made adapters on eBay for this sort of thing, or if you have a scrap lens with a K-mount you could probably make something yourself fairly simply.

I'd probably pay a maximum of £20 for the Vivitar and £40 for the Optomax on the basis that if they turn out to be a disappointment then those are the sorts of prices I would expect them to fetch me back on eBay. If you can buy them for that sort of money (or preferably less) then they'd probably be good fun to have a play with as long as you don't have unrealistic expectations regarding their image quality.

Like Barrie said above - what can you lose?

If you're doing wedding photography then you tend to need fast lenses (like your 50/1.4) as you will be shooting in a lot of dimly-lit interiors where flash isn't generally allowed. For portrait photography where you can control the lighting conditions then I'd say the lenses you have already are probably suitable to start off with. If you're creative with your reflectors then you'll get the images you want with a single off-camera flash, but you'll probably find an investment in some studio flashes will make life easier for you in the longer term.

Welcome to the forum.