Imports (things drow buy, in no particular order): material components, wood, textiles, dyes, cosmetics, foodstuffs, fruits and vegetables, preserves, grains, flour, preserved meats, fragrances, spices, metals, crafts, magical items, spells, weapons, armor, alchemical items, machinery, slaves, livestock, gems, alcohol, medicine, cultural artifacts, artwork, maps, knowledge, information, games, funiture, and various household goods.
I will point out, however, that if they're close enough to the surface for trade, then they are just as capable of raiding for many of these goods as well.
as for what we could buy from them? many of the items on the previous list qualifies, and what they may buy when their's a shortage, they'll sell when there's a surplus. of particular interest though, are knowledge of and passage through caverns surounding their city, mercenaries, ores found only in the deep underdark, liasons to other underdark races and cities (though likely not the most well-recieved), poisons, magic, arcane knowledge, spellcasting (regular magic is often hindered by the nature of being deep underground, or "underdark radiation" in you play FR) slaves, and certain other goods unique to the underground. however, its important to point out that living is hard underground and resources are sparse. what can't be simulated with underdark goods may simply not exist down there, and that could have some pretty wild cultural and socio-economic reprecussions... for instance, clay and other ceramics are probably considerably rarer underground, as such materials are uncommon at greater depths... imagine where our society may have ended up without earthenwares?