It may be economical to buy by mail, if you can avoid getting a lemon

``IBM XT-Plus - $599.'' Flip through any computer magazine and you'll find half a dozen IBM PC clones selling at the same price as Hyundai's Blue Chip, or even lower

Indeed, mail-order and local clonemakers are a burgeoning, and increasingly reliable, source of hardware and software. But buyers are advised to stay alert when dealing with these sources.

``I keep hearing more and more problems about mail order,'' says Russ Walter, a Somerville, Mass., computer consultant who offers his advisory services for free - (617) 666-2666. He is also author of ``The Secret Guide to Computers.''

The problems range from non-delivery and sneaky ads to not-so-IBM-compatible machines.

``The ad may show you an IBM clone for $499. But they don't happen to mention the monitor will cost you another $150, and it's also missing a monitor card, which will run another $100,'' Mr. Walter says.

While most people use 10 programs or less, it's worth checking on software compatibilty. Walters says a friend of his got stuck with an incompatible lemon. The company kept suggesting various solutions.

``They stalled him long enough so that when he decided to return it, they said, `Sorry, your 10-day trial period has gone by. There will be a 15 percent restocking fee to return the computer.'''

Some ads boast ``FCC approved.'' But which standard is applied? Some still sell computers that meet the outdated Federal Communications Commission Class C regulations governing the amount of electronic emissions, which can interfere with radio and television reception. Now computers are required to have the stricter Class B shielding.

Walters says service is improving at some mail-order houses. The percentage of complaints relative to the total sold is quite low. But he suggests that would-be mail-order buyers get a copy of the November issue of PC World, which provides a clone checklist.

Alternatively, he advises buying from a local clonemaker.

``I wouldn't mind spending an extra $100 to know that I could go yell at someone if there was a problem.''