I sell anything and everything! Lately I haven't been working as much as I should and I've just been trying to clear some old crap out of my warehouse, but I'll pretty much sell anything I can buy dirt cheap. I've always been very good with prices...I can look at an item and know if it'll sell at more than what I pay. When I see an underpriced item, I buy in bulk. I get a lot of my inventory from
Amazon, because they are notorious for amazing deals. Sometimes I'll find $200 items listed for $20 there, so I'll buy 100 of that item, then flip them quickly on
ebay for $100 each. The customers are happy because they got an item for half price and I'm happy because I made a huge profit. Sometimes
Amazon won't send me 100 or more of an item because it depletes their inventory, but many times I'll get everything I order. The items I sell from deals like this range from household items, to electronics, to toys and games, to DVDs, to clothing...the sky is the limit. I'll sell anything.
I used to deal a lot with Dell. I was getting very nice computers for anywhere from $30 (yes, $30!) to $225. Some of the computers were low-end ones, worth about $250...others were worth thousands. I was acquiring these computers 100% legally, but Dell hates when people make a business off of reselling their computers on
ebay, so they cut me off. I can still order from them, but it's very difficult to do in quantity. I've probably sold close to 1,000 Dell computers, but I haven't done any in the past 18 months.
Now to answer your question about timing of auctions...
Some items are VERY popular and, as such, are heavily marketed on
ebay. A perfect example is the Dell computers. There are soooooo many Dell computers listed that nobody wants to wait a week to buy from a 7-day listing when they can find 250 of the identical computers ending within one day. I listed every single Dell computer for 1-day, with a starting bid of $.99, and no reserve. I NEVER took a loss on one. To list a Dell computer for 7 days is just a waste of time, since the listing will be buried amongst 50,000 other Dell computers. Nobody will really see it until the last 4-6 hours of the auction.
Other popular electronics work the same way...iPods, common/popular digital cameras, etc. Some other items which work well on 1-day listings are popular, common, name-brand products...Kitchenaid Mixers, Heelys skate shoes, Wii/PS3/Nintendo DS/etc. game systems...pretty much anything that has soooo many listings that your item would get buried if you used a long duration. A good way to decide whether to list for a long or short duration is to do a search of current listings for the same item. If the item has many pages of listings, then it's not a huge risk to use a short duration. If only 10 of that item appear for sale, you probably want to use a longer duration...you'll have good visibilty for the entire duration of the auction since there are so few for sale, so it's in your best interest to maximize views.
One-of-a-kind, unpopular, or unusual items should always be listed for 7 days, because there's less demand for them and you want to maximize views.
Very unpopular items are best when listed as store inventory...if you list an unpopular item in an auction, it will normally sell for a fraction of what it's really worth. Letting it sit in a store takes a lot longer, but eventually somebody comes around and pays full price, or makes a decent offer using the Best Offer feature.
As for coins, I'm very new to selling them (since I'm new to collecting), but I have had no problems listing bulk ("by the pound") wheats for one-day. I would probably also have no problem selling cull silver coins for one day, since they aren't worth much more than melt value and a silver listings won't end at less than melt value. "Good" coins I'd list for 5 days or 7 days (Personally, I would choose 7 days, unless it would make the listing end on a Fri. or Sat., in which case I'd do 5 days instead...I don't like Fri. and Sat. auctions).
One thing I should point out is that I deal almost exclusively in NEW items. If I was dealing in used or broken items, I might not opt for 1-day listings (unless it's a SUPER-popular item, like the Dell Computer or Wii system). I don't deal much in used items because my belief is that time is money. If I buy 100 identical new items, then I only have to spend 5-10 minutes creating a listing ONCE, then my work is limited to clicking the RELIST button. On the other hand, every used item is unique and must be photographed, described, etc. It takes time to make EVERY listing and there's always a greater volume of customer complaints on used items than on new items. That's not to say that I don't sell used items from around my house occasionally, but I tend to shy away from buying used items for resale unless the profit potential is VERY good.
Ok, I'm rambling and offering a lot of extraneous babble that you didn't ask for, so I'll stop here!