Graffiti or street art has long left railway sidings and gone on display in art galleries, where it is avidly snapped up by trendy bankers in search of “cool” credentials. Tate Modern inaugurates its “Street Art” show this Friday, although perhaps symbolically the works remain “outside” the galleries, painted on the riverside walls. No British artists are included – curator Cedar Lewisohn sees this as an opportunity to show work from outside the UK. However, Banksy’s “Sale Ends Today” (2006), a Goya-meets-Barbara Kruger depiction of weeping women, which was included in Sotheby’s evening auction last week and estimated at a hefty £300,000-£400,000 failed to sell, indicating perhaps that this market has at last hit its ceiling. While Banksy is the best known and priciest of the graffiti artists, many more are riding on the back of the phenomenon. Banksy’s dealer Steve Lazarides has just sold out a show by Miranda Donovan (at prices from £3,000-£20,000) and, also this Friday, opens a new space at 125 Charing Cross Road. It will be packed with a “permanently evolving” show of street art priced between £1,000 and £50,000.

Everyone is fixated by powerful Russian buying in the market, but neighbouring Ukraine is increasingly making its mark. So important is this market that this weekend Sotheby’s is staging its first sale preview in Kiev, in advance of its June sales of Russian art in London. Ukrainian buyers have entered the art market more recently than Russians, but are catching up fast – one is widely believed to have paid a record £12.3m in London this February for “Grazing Horses III”, 1910, by Franz Marc. Forbes lists seven Ukrainian billionaires in its “Rich List” including the second richest, industrialist Victor Pinchuk, who owns a multi-million pound art collection including works by Koons, Hirst and Gursky and who has founded his own private museum in Kiev. In contrast, number three, Igor Kolomoisky, is also thought to be an art buyer but is highly secretive. Meanwhile, prices for Ukrainian artists are rising: for example, Konchalovsky’s “Rowan Berries on Blue”, (1947) sold for £253,000 in Kiev’s new auction house Art Kapital last December. The same work had sold in London three years earlier for just £48,000.