Damascus, Syria — Anyone who thinks Syria cannot afford to keep its Army in Lebanon for long might profitably spend a few hours at Karnak Bus Station, in the heart of Damascus.
Here the troops come home for leave or rotation. And here, the millennia-old Syrian genius for commerce is applied daily to goods of all sizes and descriptions toted back by the soldiers. There are tape cassettes. Blue jeans. Jars of honey. Imitation leather slippers. Sacks of oranges.
It is all for sale, and for bargaining, in the best traditions of the Arab marketplace. It all comes considerably cheaper than at downtown shops a few streets away. And ''it is all from Lebanon,'' confirms one young vendor matter-of-factly.
This kind of profiteering is but one element in a patchwork of costs and benefits for Syria from its military presence next door in Lebanon.
But as the United States Marines redeploy from Beirut - ''Syria's victory, Reagan's fiasco,'' one Damascus newspaper recently chortled over American fortunes there - the benefits from Syria's presence in Lebanon seem to far outweigh the costs.
Financially, the equation is pretty straightforward. Although keeping at least 40,000 troops in Lebanon with combat pay costs money, one foreign diplomat here notes, ''This is a local-currency cost.'' That is, it does not require the kind of hard-currency outlays needed for Syria's military or civilian imports.
In any case, maintaining troops in Lebanon is not reckoned to be much more costly than the likely alternative: basing them a few dozen miles eastward on Syrian soil. Other types of costs and benefits are more complicated to assess. But again, the balance seems to favor the benefits.
Militarily, the Syrian force is less exposed than it once was. Gone is the high-profile role taken when the Syrians first rolled in to put an end to Lebanon's 1975-76 civil war. Gone, too, are the days when the Syrians were made prime targets for various Lebanese factions after the full-scale civil strife.
Why? Largely because the Syrians pulled out of the Lebanese capital of Beirut in 1982 as an Israeli invasion force approached from the south. ''Police'' duty, Syrian President Hafez Assad once termed his troops' role in Beirut.
Now the Syrians mostly restrict themselves to long-range artillery fire, Western experts say. Syrian officials deny their forces have been firing at all during the recent Lebanon turmoil, leaving it marvelously unclear just what their boys in uniform have been doing.
The main military benefit of the Syrian presence is that the troops can deny, or at least complicate, access by Israeli or other enemy forces to the east of Lebanon. This swath of valley land between the Lebanese and Syrian mountains is, quite credibly, viewed by Damascus as having crucial strategic and defense importance.
Politically, the presence of tens of thousands of Syrians in Lebanon - a force far stronger than any local military group there - affords Damascus a crucial say in the Lebanese balance of power. President Assad, by shrewdly calibrating or even switching alliances with various Lebanese groups in the past eight years, has used this say energetically.
Generally, the Syrian force also has provided the essential muscle in Assad's bid to guarantee his nation a role as a regional power whose interests cannot easily be ignored. Specifically, Assad's Lebanon force has ensured him considerable leverage over the future of rival outside forces in the Lebanese conflict - notably the armies of Israel and the United States.
True, the main catalyst for the Americans' pullback - and discussion of a similar move inside Israel - has probably been the attacks on these forces by Shiite Muslim extremists. The Syrians deny charges of involvement in these. The Americans and Israelis deny the denials.
But even without such attacks, Mideast diplomats argue, the Syrian troop presence alone has given Damascus sufficient say over Lebanese politics to guarantee the failure of attempts to arrange a US- or Israeli-mediated arrangement there contrary to Syrian interests.
''We are not (militarily) stronger than the Israeli Army, not stronger than the multinational force (MNF) in Lebanon,'' said Syria's acting information minister, Farouq Sharaa, at a recent news conference in Damascus.
But he stressed that Syrian soldiers, unlike Israelis or Americans, are Arabs. They live next door. ''We are not foreigners in Lebanon. So they - the Israelis and the MNF - can't stay as long as we do.''
''. . . .the Israelis and the MNF are in a hostile environment. . . . Sooner or later, the Israeli invaders must leave Lebanon, and the MNF also.'' (Only French troops in the MNF now remain.)
And what, if and when that happens, of the Syrians?
Most Mideast diplomats assume that a ''total withdrawal'' by either of the main outside powers in Lebanon - Syria and Israel - will not happen any time soon. A ''simultaneous'' pullout - the policy aim of the Americans and of the US-backed government of Lebanon - is out of the question. Syria argues this would unfairly ''equate'' the status of Syrian and Israeli troops next door - although the Syrian force entered with an Arab League mandate, while Israel invaded.
A more likely scenario, diplomats say, is a partial Israeli pullback in southern Lebanon that would leave Israelis in the southeast sector closest to Syria, and Syrians in control of the rest of eastern Lebanon. Within this framework, internationally-mediated security arrangements might be possible, with the aim of reducing the danger of an Israeli-Syrian confrontation.
Again, Syria would veto the idea of any direct agreement with ''the Israeli invader'' - or any indirect understanding implying equal status for the two forces. Yet this need not exclude, foreign diplomats say, the kind of UN force that the US arranged on the Israeli-occupied Golan Heights after the 1973 war.
Acting Information Minister Sharaa, asked at his news conference whether a Golan-style arrangement would be as unacceptable to Syria as those so far pushed by the US, remarked: ''That's a different case.''