The inventory of pre-owned business jets contracted further in June, to 13.2 percent, down 0.3 percentage points from the year-ago period, according to data released on Friday by business aviation research firm JetNet.

Despite this news, “Business jets are showing a slow start in the first six months,” the company said. There has been a 7.8-percent decrease in pre-owned business jet sale transactions in the first half of the year and, at 402 days on the market, aircraft are now taking 28 more days to sell than last year. Average asking prices, however, rose by 4.2 percent, to $4.683 million, marking the second consecutive monthly increase.

Meanwhile, pre-owned business turboprop inventory fell by 1.2 points, to 7.7 percent, from June 2012, signaling a more entrenched seller’s market. Though sales transactions in the first half of the year were off by 7.5 percent from last year, the number of days on the market shrunk by 30 days, to 313, while asking prices saw a 14.4-percent increase to $1.431 million, JetNet said.

Pre-owned turbine helicopter inventory remained flat year-over-year at 6.2 percent, while sales transactions fell by 19 percent. Average asking prices dropped by 26.9 percent, to $1.114 million, though the number of days on the market fell by 32, to 393.