A new chapter on ‘how to sample’ popped up in my Google alerts and the chapter contains some information about sampling on MTurk. For example:
“The issue (of generalizability), as with all non- probability samples, is that those taking the survey are unlikely to be representative of the population of inference. For example, those working on the Mechanical Turk site are likely to be younger, more computer literate, and of a lower socioeconomic strata than the general population.Furthermore, on sites like this, it is difficult to limit the respondents to a specific geographic location or nationality.”
Regarding the statement in bold above: it may be sort of difficult but it just takes a few clicks. That seems a bit of an outdated concern.
“Sampling Methods for ONline Samples” by R. D. Fricker is available here: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.720.4424&rep=rep1&type=pdf
If you need a good primer on sampling, this is probably as good as any out there.