I have had somewhat mixed results. With online auctions such as DelCampe and
ebay, I have had the most problems with sellers from Spain, Portugal, and Italy but it's impossible to generalize. If you work with a dealer overseas, stick to the largest ones with the greatest presence. You won't find any bargains but they do have material that is very hard to find in the US, and they will be much more reliable overall. I ordered some stamps from PhilDom within the past year which got stolen at the Spanish post office. PhilDom refunded my entire paypal payment immediately.
Might be able to help more if I knew which areas you are interested in?