Well assuming a Loyalist Chapter, if the Chapter in question found out you'd given over one of the most sacred things they posses (gene-seed being the literal future of the Chapter) to the Inquisition, they'd probably be less than pleased unless the Inquisition immediately handed it back. Depending on the Chapter, that displeasure could be communicated as anything from a strongly-worded letter to a full scale military response.
There's also little reason for the Inquisition as a whole to want it anyway. The Adeptus Terra store gene-seed samples from every Chapter that's been Founded since the Horus Heresy, with a few exceptions. So they'd probably just arrange for its return.
Now, an individual -and massively heretical, but we'll get to that- Inquisitor could have some interest in acquiring a viable gene-seed sample. Provided he had a competent enough set of associates, including highly skilled and unscrupulous Adeptus Mechanicus Techpriests and Chirurgeons, he could potentially create his own 'Space Marine'. A tame 8-foot tall, gene-enhanced killing machine would be a valuable asset to our hypothetical heretic, though I imagine that the process could very easily go wrong without the skills of true Space Marine Apothecaries carrying out the process.
Traitor gene-seed presents an interesting case on its own, and requires some interpretation of the lore. It's possible that the gene-seed of the Traitor Legions is still maintained by the Adeptus Terra, and there are rumours that Chapters like the Minotaurs or the Blood Ravens were in fact Founded from the gene-seed of traitors. If they don't, then a sample of Traitor gene-seed could be an invaluable resource to the Imperium. Of course, if a Traitor Legion discovered that you had their gene-seed in your possession, I imagine the consequences would be even more dire than a Loyalist Chapter...
Power armour, its status as a relic of war aside, isn't much good to the Inquisition. They have ready access to power armour already, and it's no use to anyone who isn't a Space Marine anyway. They'd probably just return it to the Chapter.
What you don't mention is who you are. If you're a bonded agent of the Inquisition, then it's your duty to turn your discoveries immediately over to your master for them to deal with. Or don't...
If you're a Rogue Trader, then I suppose you could try and sell it. I'm not sure who'd buy something so esoteric and dangerous to have, beyond heretics and hereteks of various stripes. You'd probably be better off just handing it over to the relevant Chapter or the Inquisition and using the kudos gained to your advantage.
Edit: Realised I forgot to address the 'where you found it, issue', so:
There's not a lot of information on how gene-seed works, or even what it actually is. We know it's found in -or extracted from- the Progenoid Glands located in a Marine's neck and chest, and that it's similar to an egg or sperm cell.
According to Lexicanum, a Space Marine's gene-seed can be rendered sterile by high concentrations of radiation, though they don't cite a specific source for this. It gels with the idea of them being similar to reproductive cells though. So if you found it somewhere high in ambient radiation, it's probably of little use to the Chapter except as a matter of pride.
There's no source for how fast a Space Marine's tissue degrades after death that I can find, but we can probably assume that extraction needs to be pretty soon after death to keep the gene-seed viable, like any other organ transplant, otherwise Apothecaries wouldn't need to do it mid-battle unless it was a losing fight - they could always come back later. With that in mind, if the gene-seed you found is still inside the Marine, and he's been dead for a while, it's again unlikely to be viable.
If you managed to find some in storage, perhaps in a med-lab on a derelict ship, that would be much more valuable.
Finding it on a Space Hulk is an interesting proposition. It's possible that it might still be viable, if preserved in some way, but Space Hulks spend a great deal of time in the warp completely unshielded. Given the catastrophic effects of warp exposure in general, making any use of gene-seed recovered from a hulk would probably fall into the category of being a Very Bad Idea.
Power Armour is less vulnerable to the ravages of time and environment. It's resistant to radiation, extremes of temperature and weather, and general wear and tear. Unless we're talking a very long period of abandonment, you could probably make use of it again with only minor repair and refit of things like exposed moving parts and cabling. The backpack is a small nuclear reactor though, so one that's been unattended for a long time might not be the safest thing ever.
The same argument against using gene-seed found in a space hulk can be applied to power armour as well - don't wear things that have been exposed to the raw power of the warp, kids.