It doesnt matter if your crab picked it all off... its actually his job to clean up algae.
As for things to look for on LR... I try to look for pieces with good holes for things to hide in. Also I try to examine pieces for life I would like in my tank.. for example around here in FL a lot of the rock has christmas tree worms on them, if I can find a piece with lots of these OR a certain color of one.. I buy it. I also like colony worms that frequent a lot of rocks from FL.... if I see rocks with those I snatch them up so fast. Like mudplayer said.. get light weighted rocks.. more bang for your buck. Try to avoid rocks with a lot of black on them, its dying sponge and unless you're cycling your tank.. putting a piece of rock with a lot of that on your tank can be bad news. Also its better to stay away from rocks with lots of aptasia on them, unless you feel confident in being able to eliminate them swiftly.