I am going to discuss Modern British Art because it is a popular collecting area with a plentiful supply and keyword searches should return examples of known artists' works.
If you want to see what is available in this area here are some relevant names to search for: Henry Moore, Jacob Kramer, Augustus John, Duncan Grant, Mark Gertler, Stanley Spencer, Dora Carrington, Vanessa Bell.
Do a phrase search in titles and descriptions with an artist's name in quotes: eg "Mark Gertler".
In this case all the results should be Mark Gertler items. If another artist's work appears in the list then a seller is possibly KEYWORD SPAMMING, luring bidders into a dangerous auction they should not have found with the search used.
Serious collectors looking for bona fide auctions of a particular artist's work should use title-only keyword searches using the artist's name; a bona fide auction of an art work by Henry Moore will be named as such in the title.
Looking for underpriced works by known artists among anonymous or incorrectly identified works involves protracted category browsing and sophisticated keyword searches without personal names. Remember that if no claim is made about the creator of a painting in the title or description the seller can't be blamed if you make a mistake about the artist's identity.
Illiteracy in an auction description, by itself, is not a reason for avoiding an item. If an important keyword is misspelled you might be the only bidder who finds an item. However, art dealers and collectors are usually highly educated and you don't expect to see, e.g., " a auction" or "a artist" (examples found on eBay in several auctions from the same seller). So bad spelling and grammar should you put you on your guard but don't rule a bad speller out altogether; art fraud is likely to be a "white collar" crime.
I don't go along with the theory that "if the low price is too good to be true then it probably isn't". There is a culture on eBay of competitive pricing and I sometimes find that in order to get the bidding going I need to start low. However, when a seller has a stream of ostensibly "name" art at low prices with unusual but detailed provenances consider where the pictures are coming from and what they are. Why is a famous auctioneer not handling the sale?
Often the authorship of a painting is uncertain and a conjecture is all anyone can give you, but an art dealer is exceptionally unlucky if none of his eBay listings can be unequivocally stated to be by the artist mentioned in the title or description. Look out for such prevaricating phrases as "signed by, or inscribed, Henry Moore" and divide the price by 100,000 if it is only "inscribed". Modern British artists' work is mostly still under copyright and so names cannot be applied to works without permission or good reason and incorrect attributions will offend against eBay's VERO (verified rights owners) rules.
Sometimes a listing on eBay contains a screed arguing for an attribution the experts have been too stupid or corrupt to think of, or back. Avoid; the experts are not going to change their tunes when you own the alleged masterpiece.
Art market coups are made on eBay, usually by experts.
Contemporary subversive and jokey British art by the likes of Banksy, Damien Hirst and Jamie Reid is sought for and is apparently listed on eBay but due to the nature of the material it is hard to ascertain the authenticity. "Kurt Schwitters" listings, another favourite with the forgers, can have anachronistic labels and tickets incorporated in the collages.
Especially in combination, these things should put you on your guard:
"inscribed or signed" : the seller probably knows it is not a genuine signature because he would say "signed" if it was, and with a well-known artist, he will have had it checked. "Inscribed" means someone has written the name on the item.
Spam: in a spammed proper name search return you've been tricked into looking at A when you went looking for B. What other tricks has the seller up his sleeve?
Too copious supply: is he pilfering a collection? I remember a seller who was listing mouth-watering early pottery such as slipware with seventeenth century dates and very modest prices. When I checked back a month later the seller was Not A Registered User; very suspicious.
Red herrings: an over-long description. It is larded with hints to allow you to draw your own wrong conclusions and you don't notice he has misssed out something vital. A listing that is wearying to read will not be read in full by many bidders who are thereby caught out by "small print".
Where is it from?: different kinds of sellers acquire stock in different ways but the sellers with scores of name-artist listings should be able to supply provenances better than "found in a pension near Vallauris the owner said was popular with artists in the 1930s", "in a job lot in a sale which included a signed Matisse drawing" (yawn).
Disclaimers: the seller's items have "get out" clauses and in no case is he unequivocal about the authorship.