This is a broad subject and impossible to cover in a few paragraphs. I'm not talking about digital cameras, but instead the common SLR (Single Lens Reflex) 35MM cameras made between 1920-1985. Though most people consider these cameras obsolete, some are still very collectible and can command thousands of dollars.
Most of what you'll come across at sales are the 1970s -1980s cameras like Canon, Pentax, Minolta, and Olympus. Great names in their day but as a rule, most of the common entry level bodies with 50MM lenses sell for under $50. on the secondary market.
For example, the Pentax K-1000 35MM in good clean working order can be bought online between 20.00 and 50.00 I see them over priced in shops, with some extra accessories, for 150.00 and more. They are also still sitting there collecting dust when I go back 2 months later. The Minolta, Canon, and Pentax cameras that are functioning properly with clear lenses should be bought for resale 10 bucks or less...Really. Some people think a lot of these cameras and would be insulted by an offer of ten bucks but the reality is, they just don't bring much anymore.
Below are a few examples of $10 to $50. (Camera, case and lens only)
Try not to pay more than $5 or $10 for any of these in clean working condition. If they have ANY issues, pass on them unless they come with a bag full of lenses and filters. Market not strong on these.
Pentax K 1000 $10 $50 .
Yashica FX-2 $10-$50
Minolta SR t 101 $20 $50
Canon AE-1 $5-$30
Now for the opposite end of the spectrum. Say "BIG cheese!"
Most of the better name in photographic equipment came out of Germany. The important name not to be confused or compared with the cameras above is Leica. Some of the rare models still bring 10,000 to 18,000 in minty condition. There are even confirmed sales over 20,000 for the Leica 1 A Anastigmat f3 of which there are only 150 in existence.
The main two rules that apply here are:
1. Like new to excellent condition on common cameras.
2. Rare cameras should be acquired regardless of condition though poor and damaged will factor into the pricing. Rare Leica models have seen their values increase disproportionately to common ones.
Things to look for: That little hockey stick shaped piece of metal on the left side of the lens when you're facing the camera. If you're lucky enough to find one of these at a garage sale and the owner recites the story of how much grandpa admired it because his dad gave it to him when he was a kid...Home run! "That's been in my family since the 1930's"
The early cameras had a simple rectangular streamline bodies with 50mm lenses. If you do pick up one of these rare models, don't rush to dump it on Ebay. Do your research. Serial numbers and lenses make a world of difference in the price. They are in such demand there are copies of rare cameras out there. Contact a professional and get at least two opinions. Dealers will show immediate interest and may offer a high price. Don't be tempted by the first offer. It's amazing what collectors will sometimes offer beyond book value.
Below are a few examples of Leica:
Leica 111B $900.
Leica 1 A Elmar Fu 50MM 1926-30 $1500 - $4500
Leica 1 (B) Compur with Elmar $3000-$7000
Leica 111g 1956-60 $800-$2200 Leica
As stated condition is everything. Collectors and dealers often first ask if the lenses have mold in them. This is a problem with all older cameras if stored in area of extreme temperature swings. Look for spots on the inside on the lens when back lit. If sandwiched between the lens, it will have to be professionally corrected.