Could I use the Mann-Kendall Trend test for a data set of monthly sales (counts)?
If your data are collected monthly over several years, what you might need is the seasonal Kendall test - a version of the Mann-Kendall test for detecting monotonic trends in seasonal data (where seasonal refers to monthly in your case). The seasonal Kendall test runs a separate Mann-Kendall trend test on each of months you have data for and computes an overall test statistic by adding up the monthly Mann-Kendall test statistics. Any monotonic trends present should all be in the same direction (either all up or all down). If the trend is up in some months and down in other months, the results of the seasonal Kendall test will be misleading. See https://pubs.usgs.gov/twri/twri4a3/pdf/chapter12.pdf.
If your data are collected monthly for a single year, then a Mann-Kendall test might be fine.