ANYONE WHO DOUBTS THAT THE trickiest phase of an initial public offering is the very last one -- pricing the issue -- might ask Merrill Lynch, Pierce, Fenner &/or Smith. And if the country's most prominent retail brokerage house deigns not to comment, try Roy M. Speer & Lowell W. Paxson of Florida-based Home Shopping Network Inc. (HSN). This spring the treasury of HSN, which sells gewgaws over cable TV, took in about $30 million through an IPO that Merrill Lynch Capital Markets managed -- a reasonable piece of change, it seemed to everyone at the time -- everyone, that is, except investors.

HSN was brought public on Tuesday, May 13, 1986, at $18 a share, and that first day closed at $42.63. On Friday, the fourth day of trading, it was $53.25. On June 20, after only four weeks of public life, HSN declared a three-for-one split -- possibly the earliest such announcement on record. And before the month was out, the stock hit $108. Granted, underwriters have been tending lately to leave something on the table. But not a whole meal.

The upside potential that eluded Messrs. Merrill et al is the impact that slick TV merchandisers such as HSN are about to have on the old order. Mail order, for instance. Over the decades, conventional catalogists have been content to print out their offerings in color brochures, curry through expensive lists of possibly long-gone customers, pay Uncle Sam a pretty penny to scatter their dog-eared line of goods dumbly onto thousands of vestibule floors, then wait for the orders to pour in. Toiling round the clock, such cable hucksters as Home Shopping Network, on the other hand, can continually reach out to a cable market estimated at more than 36 million with vivid spiels for merchandise that can be ordered by phone.

Today's shopping programs deal mostly in such obscure or discontinued items as the Adam home computer or the Nimslo 3D camera. But it looks like the revolution has barely begun. At least half a dozen formidable entrants -- Time, Warner Communications, and Financial News Network among them -- already are involved nationally, and locally originated selling programs are proliferating like pitchmen at a country fair. Given this impetus, cable selling will soon be taking on clothing, tools, camping, electronics, and other conventional catalog collections. And as two-way telecommunication technology itself advances, it could march all the way through the halls of traditional retailing to the last bastion of eyeball-to-eyeball salesmanship, the automobile showroom itself.