Welcome to the forum, Jonathan.
If you don't know yet, all Pentax DSLR camera will work with these lenses, in case you are in the market for buying a DSLR and were interested in the possibility of being able to use your existing lenses.
If you look on Ebay or wherever, camera bodies really don't bring in that much money unless they are very old/rare/special, which doesn't hold true for the MZ-50, as it is a fairly common camera. If you do decide to sell it, make sure you check that the mirror works and doesn't get stuck- that is a common problem with these cameras that happens over time.
The 28-80mm lens doesn't really ring much either because it is fairly common ---->
SMC Pentax-FA 28-80mm F3.5-5.6 Reviews - FA Zoom Lenses - Pentax Lens Reviews & Lens Database
I'm not very familiar with the Sigma, but here it is in the lens database ---->
Sigma 28-300mm F3.5-6.3 DG Macro (AF Lens) Lens Reviews - Sigma Lenses - Pentax Lens Review Database
Looks like the average price paid was about $240. A quick Ebay search yields some results --->
sigma 28-300 pentax | eBay
If I were you, I would keep the MZ-50 and 28-80mm lens together and then see how much you can get for the Sigma separately.
You could list them on Ebay (make sure you have good pictures of the items, a good description, and a reserve price just in case), but the seller fees are avoidable if you decide to sell them here in the Marketplace- to get access to it, all you have to do is donate at least $10 to Pentax Forums.
Here's some info on the Marketplace here on the site, if you're interested:
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https://www.pentaxforums.com/forums/photographic-equipment-sale/137-read-firs...ack-guide.html
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https://www.pentaxforums.com/forums/photographic-equipment-sale/