I was an Industrial Photographer and Photojournalist from 1968-1980 and now that I am retired from another industry would like to get on the stock market trail. I realize the digital game has changed everything, so without any digital equipment, I am trying to find out if there is any bridge or used DSLR (w/lens) under $300 that is good enough for stock work.

I would say it really depends on what you want to photograph for stock.  With the caveat that I'm a newbie with a small port, I've uploaded and sold photos taken on a Canon G12.  Most of my stock photography is from a Canon 550D (Rebel T2i if you're American, Kiss 2 if you're Japanese), much of that with the kit 18-55mm lens.  Any reasonable camera will do just fine if you use it right.  As I understand it, the Canon 450D upwards are all good quality for your money, and I can definitely vouch for the 550D.

I was an Industrial Photographer and Photojournalist from 1968-1980 and now that I am retired from another industry would like to get on the stock market trail. I realize the digital game has changed everything, so without any digital equipment, I am trying to find out if there is any bridge or used DSLR (w/lens) under $300 that is good enough for stock work.

Getting into stock will take more of an investment than that.  Micro (and macro) stock shooters are using top of the line pro-sumer cameras, like the 5dMk3 which make it easier to get great quality images.  You're going to need to be at that level to compete.  Plus, you'll need Photoshop or similar, and a good, fast computer.

"Good enough" isn't really good enough.  Unless you want to shoot with your phone and go for the mobile look.

I had plenty of success with a Nikon D80.   The only real drawback is that at 10 MP, some sites will limit the maximum size you can sell.    Unless you're doing something like night shots, where sensor noise becomes a problem,  a used D80 would be fine.   Or for a few more bucks, a D90 is better.   Stock agencies obsess over noise and focus and will inspect at 100%, but you don't need a cutting-edge DSLR to meet those requirements, just some understanding of ISO vs noise, noise reduction and sharpenting.

No, it's a bit of both.

Great source of used gear: http://www.fredmiranda.com/forum/board/10

I have had images accepted and sold quite a few with a lowly Canon A620 years ago, but they have to be with perfect conditions. I also have a G10 that a few have been accepted and have been selling, but those are old cameras and I only use them for fun now. I've been using Canon Rebels since the original one came out and now have the Canon 3ti and several lenses. I would not buy used equipment unless it's something that is still being sold as new. If you want to keep the price down, invest in a Canon 3ti or 4ti. They will last you many years. Maybe start with the kit lens (I don't care what you guys say, you can get images accepted with the kit lens) and maybe a macro lens. Get used to the process of uploading and keywording and see how productive you are or if you can make money at it. If the money starts coming, then upgrade to professional equipment.

If you are not experienced in the use of Photoshop, I think you will find that processing the image to get results that are competitive with other images on sale is at least as hard as getting acceptable image quality from the camera.