Politeness goes a long way here as everywhere else, so it depends more on your way of asking then the question itself I’d say. Most likely, condition and originality are the two points we can help you with the best. For price comparisons, while it’s ok to ask, may I also point you to the eBay advanced search? There you’ll find the option to click “Sold articles” - this way you’ll see for what prices they
actually sold for, as opposed to what people are dreaming to get for them. This works best if you have the reference number of the watch in question. Let’s say you’d like a Seamaster 2846, one of the (if not the) model produced by Omega in the largest number in the 50s. So you search for “Omega Seamaster 2846” on eBay, limit the search to sold articles and then try to find whatever models compare best to the one you’re looking to possibly purchase.
The forum search is also a handy tool for that, you can use it to scroll through relevant ads in the classifieds here - they all still show the asking price, even if they might be sold already. Do note though that prices have increased significantly in the last half decade or so. A priced realized in 2015 might be unrealistic to hope for today.
Enjoy the hunt.
😀