The most overused descriptive word in online philatelic and postal history cover sale descriptions is a single misused word: rare. First runner up is the term seldom seen, which always should prompt the buyer's question "By whom?" And for that nebulous hard to find item, you might want to wonder "Then how did I find it so easily?"
Related to this is limited edition. Stop and think; everything is limited. Those descriptions that use that term and don't say what the issue is limited to should be questioned. Contact the seller to find what the limitation is. If he doesn't have an answer, reconsider the purchase.
Fudging condition descriptions is a common problem in online sales. Damaged items are described as fine; faulty (not quite damaged) items are "in excellent condition" with no mention made of the faults; covers are erroneously described as "mint," stamps as "uncirculated" and hype words like treasure, gold mine, and heirloom quality abound. Make sure you buy an item that features a good picture and a realistic description to avoid pain and buyer's remorse after the fact. A bit of honesty in a description is a welcome thing, although something like "quite dirty cover, but doesn't take away from its appeal," perhaps a miracle of mixed message, wouldn't seem to help gain a sale.
Tech Allows Modified FDCs
There is currently a great number of add on cachets appearing on the market. The trend is the product of putting a print shop in everyone's computer, allowing these after-the-fact cachets to be added to uncacheted classic first day covers. Beware - they are not always clearly identified as such and can fool the unaware buyer.
A big problem is the term unknown cachet left at just that without any explanation that it might be a modern add-on. Some are made to look like the era of the stamp. One doesn't want to go throwing the term fraud around, but these cover descriptions rub right up against the definition. It is understandable when a description bases the "unknown" on the fact that the cachet doesn't appear in any of the ID catalogs for classic cachets like Planty and Mellone. But if it is a modern add-on, that wouldn't appear in a classic catalog in any event. So the description means nothing. In short: classic unknown=rare; modern add-on unknown cachet=potentially plentiful and worthless.
Don't Be Dazzled by Covers With Artistic Pretensions
The term hand painted is attached to all sorts of cacheted covers by those who are unaware of the proper use of the term. One should be uncomfortable, if not suspicious when one sees the term online. One recent egregious example seen online was a crude pencil drawing on a youth-related first day cover from the 40's. In any dealer bargain box at any stamp show, you would find this item for $1.00 and think twice about paying that much. The price in the online first day cover dealers list? $22.50!
Final Cover Warnings
Stock Photos - When a cover is pictured on eBay and is a stock picture, some of the buyers object. (Obviously there can be no stock picture for a unique or unusual cover.) This is a fine line. How much can a first day cover vary? I understand the complaint if the cachet is different. But if it is merely a slight difference in the FDOI cancel, it is really a foul? Granted, stamp and cover collectors can be very particular - if one falls in love with the pictured item, receiving another can be a deal breaker.
Unusually High Prices - There must be something about cachets of popular design that make those not familiar with fdc's to put astronomical prices on items worth no more than a couple dollars. These can frequently be found offered in online auctions where stamps are not a priority of the seller. If you have a doubt about the price, do a search of similar items to make a comparison. If the price is wildly out of line, you'll obviously take your business elsewhere.
A Bad Box of Covers - When one wants to sell a variety of covers, one must take them out of the box and post individual pictures. Taking a shot of the first cover and then the tops of the rest gives the buyer no idea what the seller is attempting to sell, beyond the basic description like "airmail covers." This happens more often than you think, and while the buyer can contact the seller for further description, putting the burden on a potential buyer is a bad way to do business, to say the least.