Antique shops are run on a commercial basis and I think they often expect customers to make sensible offers on items and if they have been in stock a while then there is a chance they will accept a lower price with maybe a bit of haggling.
There is no harm in going in, showing interest and then make an offer. I don't think the proprietor will be offended. Often they buy in items at rock bottom prices and impose a high mark-up and at the end of the day they just want to turn a profit.